So in Thief, you're a Thief.
Despite the misleading title of the game, however, shadows do not actually harm you in any way. Rather, it is advised to stick to the shadows as much as possible. I did not know this, and so had a very troublesome first few hours of gameplay.
In Thief, you steal things. That is because you are a thief. When you steal things, they belong to you, and so you can sell them to people and get money to buy other things that make stealing things easier. With this unique system in place, you can achieve a near-constant cycle of owning and disowning various objects in the game world. It is systems like these that contribute much of the immersion to Thief: Not Really Deadly Shadows.
You can posess a myriad of useful tools, like a knife. That is sharp, and so it can be used to stab guards, often causing them to die. While to some, like the guards, this may be seen as unhelpful, for a thief, it is actually quite useful.
With well thought-out game systems, a very well implemented loot ownership gameplay style, and the ability to own things, I highly recommend this game with something like a 7 or 12 out of 10.
"I would own this any day of the week, and you should too"
"This is fun"
"I love owning things"
"I love disowning things"
"I would disown this game just to reown it"