Every profession in Guild Wars 2 has some unique abilities that set them apart from other professions, and in the case of the Thief, that unique talent is the ability to steal. The Thief's ability to steal allows the player to steal something from an opponent and then use that item. Most of the time, the item gained is some form of weapon that can be used against the opponent, but sometimes it can be in the form of a boost.
To steal, the Thief activates the ability by pressing F1, which allows the Thief to shadowstep up to the foe and steal the item. The range for stealing is a healthy 900 with a cooldown of 45 seconds. Once you've stolen from a foe, you can use the item gained by hitting F1 again. Each creature that you face in Guild Wars 2 seems to have certain items associated with them. For example: moa birds tend to give feathers that you throw or eggs you can eat. Please note that you don't have to use the item you've stolen right away. You can store it for later use, but you'll be unable to steal again until you've used the item you're holding. However, once you've stolen from an opponent, they are aware of you so be prepared to start throwing down.
Stealing is a fun mechanic for Thieves and introduces a random element into combat. You should definitely be attempting to steal as often as possible as that the effects from the various items gained can be quite beneficial, such as causing blindness, bleeding, or going into stealth. Experiment with this ability and see what you can steal from the various creatures in the game. Also take heed that quite a number of stolen items have a decent range associated with them, like rifle shot or throw feathers.
The items that you can steal as a Thief vary quite a bit as you can see from the miscellaneous items shown below.
All in all, the ability to steal is a fun, and useful, ability for a Thief in Guild Wars 2. You automatically gain something useful to use in combat and it costs nothing in initiative to use. Plus, you're a Thief so you should be stealing from your enemies whenever you can!