A Shardblade is a powerful weapon capable of slicing rock and severing souls. People who possess a Shardblade are called Shardbearers; they are soul-bound to each their Blades until death, or until each Blade is willingly relinquished.
Shardblades were devised as imitations of the Honorblades, which were created and given to the Heralds. So, since the original pattern was the Honorblades, they were built to feel like the Honorblades.
Honorblades are not spren. Just how they were created is not known, but they came from Honor. Later, certain spren learned to imitate what the Honorblades did by forming Nahel bonds with people with cracked Spiritwebs. When those people betrayed their oaths, the spren died and became regular Shardblades.
Shardblades are generally associated with Shardplate; a suit of armor that completes the set.
According to legend, the Shardblades were first carried by the Knights Radiant uncounted ages ago. Gifts of their God, granted to allow them to fight horrors of rock and flame, dozens of feet tall, foes whose eyes burned with hatred. The Voidbringers. When your foe had skin as hard as stone itself, steel was useless. Something supernatural was required. 
Four millennia later, they are regarded as priceless relics. Wars are fought for, and won by them.
Within his visions, Dalinar was shown roughly 300 Shardblades that were abandoned by members of two Radiant Orders. Dalinar estimated, at the time of the War of Reckoning, that there were less than 100 Shardblades in all of the world. Alethkar and Jah Keved own some twenty Shardblades each; the rest are spread amongst the weaker nations, including five Shardblades held by the Thaylen royal guards. Selay, and other kingdoms, such as Herdaz, have only a single Shardblade apiece. Kharbranth and the Unkalaki people are not known to have any Shardblades.
So far, no amount of fabrial science has even approached the possibility of re-creating Shardblades. However, recent rumors have claimed that Alethi scholars are close to creating new Shardblades.
No two Shardblades are the same, but all Shardblades cut easily through most inanimate matter, so long as the Blade is kept in motion. They do not cut living flesh; instead the metal fuzzes as it passes through, killing without leaving a mark or spilling blood. It was said that Shardblades sever the soul itself, the only indication of death being that the victim's eyes burned. Any part of the body that is cut dies instantly. If the Blade touched a man's spine, he died, eyes burning. If it cut through the core of a limb, it killed that limb, rendering that limb permanently limp, numb, and useless. A severed Shardblade limb needs repair to the soul before it would function again. However, Surgebinders can heal limbs severed by Shardblades by using Stormlight. If a vital organ is struck, it will render that organ useless as well. However, once dead, flesh can be cut just like any other object. Also, parts of a living body that are made of non-living tissue such as hair, fingernails, and the shells of beasts are cut normally. A modern interpretation is that Shardblades can only sever nerves.
Shardblades were enormous, yet remarkably light. They were designed to fight something larger than another person. The large weapons took a great deal of skill to swing properly. There was never recoil; landing a blow felt nearly like passing the Blade through the air itself. The trick was to control momentum and keep the Blade moving. 
A Shardblade could only be blocked by another Shardblade, by Shardplate or Half-shard shields.
A Shardblade is summoned by the intent of the Shardbearer, willing it into existence, typically dropping into his/her hand extended to the side. The process of summoning takes exactly ten heartbeats. When it materializes, the Blade is typically seen covered in water beads along its length, as if condensed from mist.. However ...
|“||So the more urgent you felt, the sooner you were armed.||”|
So, the summoner's heartrate definitely has something to do with the summoning of his/her Shardblade.
If a Shardbearer lets go of his/her Blade, it disappears unless it was willed to stay (which was a protection from being taken by the enemy during battle).
Shardbearers are soul-bound to their Blades until death, or until their Blades are willingly relinquished. The process for realignment first requires the new Bearer to simply pick up the Blade. Second, upon acquiring the Blade, it takes five days before the Shardbearer fully bonds to it (and is, otherwise, able to dismiss it). When the Shardbearer dies, the Blade rematerializes next to him/her, allowing anyone else to pick up the Blade and become its new owner.
Though each Shardblade is unique, there are in general three types of Shardblades.
The first is the type that is wielded by some Sharbearers in The Way of Kings and Words of Radiance; they take ten heartbeats to summon, have a gemstone in the pommel, and can cut through stone easily. When held by someone bonded to a spren, screams fill their wielders' heads. (Syl mentioned that these types of spren are dead - since the first Order of the Knights Radiant abandoned humankind - and that this is why people who are bonded to their Blades hear screams.)
The second type are those wielded by Knights Radiant; they can be summoned when the spren are ready and can take any form (i.e., Syl could be transformed into a spear, dagger, halberd, and/or shield during Kaladin's mid-air duel with Szeth). They have the same properties as other Shardblades, but unlike other blades, they glow certain colors from glyphs and symbols on the blade, and their bearers hear no screams from them.
The last type, with regard to Szeth's (now previous) Blade and Taln's (now previous) Blade, are actually Honorblades. Szeth was never bonded with a spren. Taln was one of the Heralds who wielded an Honorblade, yet lost it in his present madness. (In Words of Radiance, once Dalinar had become a Radiant, he heard the Blade he held screaming, indicating that it was a dead-spren Shardblade.) Honorblades are believed to grant some power over the paired Surges (i.e., Jezrien's Blade grants Adhesion and Gravity; Nale's Blade grants Gravity and Division, etc.). These Blades were made by the Almighty himself and give the power of the Radiants to whomever wields them.
|“||There is something wrong with your Blade, and with all Blades. She hesitated for just a second. All but mine. Pattern!||”|
|“|| So they're all spren, he said. Shardblades.
Syl grew solemn.
Dead spren, Kaladin added.
Dead, Syl agreed. Then they live again a little when someone summons them, syncing a heartbeat to their essence. 
|“||We're spren, Syl said. We're forces. You can't kill us completely. ||”|
|Shardbearer||Name of Shardblade||Description||Current Status|
|Dalinar Kholin||Oathbringer||Six feet long from tip to hilt. Long and slightly curved. A handspan wide. Wavelike serrations near the hilt. Curved at the tip like a fisherman's hook.||Hidden in Urithiru|
|Sadeas||Oathbringer||Formerly Dalinar's Blade.||Hidden in Urithiru|
|Elhokar Kholin||Sunraiser||Long and thin with a large crossguard. Etched up the sides with the ten fundamental glyphs.||With Holder|
|Gavilar Kholin||Firestorm||Six feet long with a design along the blade like burning flames. Silvery metal that gleamed and almost seemed to glow.||Unknown|
|Kaladin||Sylphrena||Kaladin's Shardblade is his bound spren, Sylphrena. She can take any form that Kaladin needs her to assume. In his fight with Szeth, she takes the shape of a sword, shield, spear, halberd, dagger, and warhammer. Anything he wants or needs her to be, she will be.||Alive with Holder|
|Shallan Davar||Pattern||Like Kaladin's, Shallan's Shardblade is the bound spren, Pattern. He does not require any time to come straight to her hand.||Alive with Holder|
|Szeth||Unknown||Long and thin. Edged on both sides. Smaller than most others.||With Kaladin at Urithiru|
|Adolin Kholin||Unknown||Blue, one-edged blade. Sharp side is sinuous, rippling like an eel curving up into a point. Opposite side has delicate ridges, with the appearance of crystal formations. Six feet long.||With Holder|
|Amaram||Unknown||As tall as a man, curved or sinuous, unsharpened side ridged with flowing waves of flame, etchings along the length, gemstone at pommel.||With Holder|
|Eshonai||Unknown||Wicked and barbed like flames.||Presumed to be with Holder|
|King Hanavanar of Jah Keved||Unknown||Unknown|
| Shardbearer guard #1|
from Jah Keved
| Shardbearer guard #2|
from Jah Keved
|“||Ten heartbeats, Return to me, you creation of Damnation.||”|
- It has been theorized that they are stored in the Spiritual Realm, when not used.
Q. If a Shardblade was used on an awakened object, would it physically harm the object, destroy the Breath, or a combination of the two?
A. You will someday know.
Q. Is there any ramifications to the holder of a Shardblade for using a blade in a manner that it wasn't intended?
A. Depends on the type of Shardblade. (You have seen three different kinds in TWoK.) For most, no. For some, most certainly.
Q. Is it possible then to reawaken a Shardblade if that Blade is wielded by someone who speaks the oaths of a Knight Radiant?
A. Yes, but it would be extremely difficult. The spren in a Shardblade are not trapped in a state of mid-transformation like the Elantrians. They are stuck in an agony cycle after having a significant portion of their consciousnesses ripped out of them. The Nahel bond is what allows spren to think on [the] material plane and that has been torn away. It would be like having a data jack installed and then having someone come up to your head and rip it out of your head.
- ↑ Imitations ...
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 The Way of Kings, Prologue - To Kill
- ↑ The Way of Kings, 29. Errorgance
- ↑ The Way of Kings, 23. Many Uses
- ↑ The Way of Kings, 12. Unity
- ↑ Words of Radiance, 52. Into The Sky
- ↑ Severed limbs ...
- ↑ 8.0 8.1 The Way of Kings, 58. The Journey
- ↑ The Way of Kings, 56. That Storming Book
- ↑ The Way of Kings, 26. Stillness
- ↑ 11.0 11.1 Words of Radiance, 89. The Four
- ↑ 12.0 12.1 12.2 Words of Radiance, 87. The Riddens
- ↑ 13.0 13.1 13.2 13.3 Words of Radiance, 86. Patterns Of Light
- ↑ 14.0 14.1 14.2 The Way of Kings, 13. Ten Heartbeats
- ↑ 15.0 15.1 The Way of Kings, 69. Justice
- ↑ 16.0 16.1 The Way of Kings, 46. Child of Tanavast
- ↑ The Way of Kings, 68. Eshonai
- ↑ The Way of Kings, 1. Stormblessed
- ↑ The Way of Kings, 47. Stormblessings
- ↑ The Way of Kings, 51. Sas Nahn
- ↑ 21.0 21.1 The Way of Kings, Inter9.
- ↑ 22.0 22.1 The Way of Kings, 9. Damnation
- ↑ Shardblade vs. Awakened object
- ↑ Types of Blades