Combat Arms is an online FPS game with a lot of customizable features. In a nutshell, it is similar to Call of Duty, both in game mechanics and appearance. You battle it out against other players in various game modes and you can buy new weapons and some are even upgradable or customizable. The graphics though are a bit out-of-date, although the game mechanics are impressive.
Soon after you've entered the game, a window will pop up asking if you want a tutorial. If you've ever played a FPS game before I suggest you skip it, because it only explains basic movement and shooting. To join a game you first need to select a server, then a channel and only after that you'll be able to see the available matches you can join. The game also has a quick start feature, where you can select some custom options and it will automatically put you in a match that coincides with your preferences. The game offers a really huge variety of maps, so you'll always have a change in scenery, which is nice. Also there are 10 game modes you can choose from. Probably the greatest feature Combat Arms has, is the large variety of weapons and other items. You start the game with a decent amount of in-game money, so you can buy new weapons right away. However you don't really buy weapons, it's more like a renting system, because you buy them for a set amount of days. After that, you'll need to buy them again, and again. Many weapons are customizable and upgradable, not all though. You can put different scopes, bigger ammo clips, silencers just to name a few. There is also armor you can buy, which will enhance your HP and resistances.
The game mechanics are pretty much like in every FPS out there. Movement is done with WASD and aiming and shooting is done with the mouse. You have a set amount of HP and you won't be able to heal, much like the system in Counter-Strike. The shooting mechanism is very realistic, the game abides by the rules of physics and you need to be patient when shooting because of the recoil. If you spray all your ammo clip in one burst against an adversary, you most likely won't have good accuracy. The gameplay is fast-paced and each round will reward you with an amount of in-game money, depending on how well you did. One thing that bothered me was that I encountered quite a large amount of players using hacks. So this means that the game doesn't have a really good anti-hack system, which can be frustrating at times.
In conclusion Combat Arms is a very good online FPS game. The game mechanics are solid, it's funny to play and it has a nice rewarding system. Also it supports in-game voice chat, so you can coordinate better with your friends while playing.