Like I said, I only started yesterday so I don't know too much but....
I'll start by saying you can look at the player guide at
http://www.gunzonline.com/ for info.
Basically, this game is totally PvP and you just fight against other people. Your weapons....guns and swords (and grenades). It has a 3rd person view FPS perspective in that your objective is to kill the other guys with your weaponry in hand. Like stated above, it also has a RPGish perspective in that you assume the role of this character, and when you kill somebody you gain experience. Once you get enough experience.....guess it......you level! I know at least you get gold (or bounty) with gaining expereince/leveling, and I think you can get certain better items with your bounty and based on level (so like level 10 for a better sword or something).
The game itself is a pretty strategical one and you will probably need some skill in order to play it effectively. When you start a new character you can pick one of different "classes" if you wanna call them that; for example someone like a gun figher or whatever will have more focus on his guns and may have better guns, there is some kind of priest player that has healing packs, and there is also a scout that has smoke grenades and can be quite stealthy. There are other classes but you can investigate them yourself. There are several modes of gameplay, such as Deathmatch, Team Deathmatch, and I actually forget the other modes but they should be ones you have seen in previous FPS. In-game, the system uses a WASD-keyboard configuration to move around, with the mouse to look around and other keys such as SHIFT to block (only with sword, not guns) and space bar to jump. There are combinations of keys you can use to do some cool tricks (which I don't really know so I can't tell ya).
Overall, I could tell ya more, but I'll just let ya try it out yourself if you like the stuff you see on the website. Many players say they like the game, and probably an equal number say they don't. The only thing I should say is don't base your decision as to like it or not towards the game just because you may be a new player and see some really good players online, 'cause I know that's what I did the first day, but hey, I'm still playing it on the second day so it's not a bad game. I guess I'd give it a 7.5/10 or so, pretty good game.