Do not pass up this oppurtunity to play one of the most well-developed RPGs in a very long time.
A witcher is something of mutated monster-hunter who has the ability to fight monsters that normal humans really cannot even dare to fight. They are outcasts for the most part and so Geralt can relate to both humans and non-humans, which plays a pivotal role later in the game.
The game is played in a similar point and click combat that games like Guild Wars or WoW have. I know it seems a little bad (or maybe not, I enjoy that control scheme), but the game does more things to make it vary. There are different strategies to fighting which will tell you which sword to use, and which style to fight in. There are also signs, which are basically magic, and the one that you will most likely use most is the fire sign.
The way the game really shakes things up though is by having you make potions. Based on the difficulty you choose, potions could either have no importance or be literally and figuratively life-savers. They can do things like enhance your reflexes, increase your damage output, increase health regen, allow you to see in the dark, etc. The trick about potions though, is that each one gives you a little bit of toxicity. If you have too much toxicity in your system (shown by a bar) then Geralt will start hallucinating and the screen becomes incredibly blurry, with all sorts of things happening visually.
The game is a game of choice, with distinctly different endings and game events based on choices made by you. Ultimately, though, your goal is to kill the group Salamandra who attack you and the other witchers (I am not giving anything away, its literally the first thing that happens). How you go about it is somewhat up to you.
If you do get the game though, my recommendation is to play as much in each chapter of the game as you can, excluding the first chapter. The first chapter is very straightfoward, but 2-5 are extremely complex chapters set in a world where you make choices pretty freely and it is very easy not to get to see all of the content in the game. You really should take advantage of every quest possible, because the game is so much more fun that way. I made the mistake of speeding through the game, thinking free time later. The problem with that idea is that free time in the game is always, and there aren't really any time constraints on anything. It is similar to Oblivion in that respect, but a little bit more restricted.
Overall the game is an incredible experience. Pick up a copy because if you don't, you are missing out on one of the most engaging RPG experiences of the last many years.