Combat Arms is an FPS game that's more arcade-y than hardcore. The controls are standard, the graphics solid and the action is what you'd expect from an online shooter. It doesn't have the polish of a game that costs you $$$, but it has more going on than the rest of the free online shooters.
While there's no tutorial to get you started with Combat Arms, the game plays... pretty much exactly like every other FPS game going. It's more arcade-y than realistic though, and doesn't have the polish of games that cost you $$$ to play. Combat Arms does have the edge over the other free online games on the web though, with better graphics and more support from Nexon and Doobic.
Game modes in Combat Arms include:
One-Man Army: It's deathmatch, go pwn some n00bs.
Elimination: Team deathmatch, don't shoot your buddies, k?
Search & Destroy: Set a bomb on a checkpoint like a good little terrorist and defend it until it blows. Or, join the other team and shut the terrorists down.
Capture the Flag: It's capture the flag.

Game creators (not hosts, everything is hosted on servers) can set match specs, from weapon limitations to friendly fire and the like. This can be a bit frustrating if you've dropped your GP on a sweet sniping package and the game is an all-shovel slapfest, but there are lots of servers and games to choose from.
Some of the frills that you won't find in Combat Arms include ping info and skill-based game matchups. Joining a game can pit you against Sam Fisher's evil twin, or someone with the hand-eye coordination of a tomato. You can check soldier ranks to see how deadly the players are likely to be, but there's no way to search for games by relative skill or auto-join the same. A bit frustrating, but only slightly.

The biggest problem with Combat Arms is the hacking. There are cheat patches available online that let you strap on some tights and slap a giant 'S' on your chest as they grant you invulnerability, no clipping, flight, one-shot kills, etc... It's the kind of thing that can make you wish someone would jam an awkward and lumpy fistfull 'o kryptonite squarely up the backside of the offending player. Thankfully, it's pretty rare so you won't have to worry about spandex-clad players much. If you are concerned, get involved in a clan and start playing clan games where it matters more if players are reported for hacking.
Next up we have a few words with Nexon, the developers of Combat Arms, about a cool new game mode. We also have word about some secret developments.