When you spawn in Frontlines, you've got two decisions to make about what sort of character you're going to be. Your Loadout decides what set of weapons you're going to get - with Sniper, Close Combat, Special Forces and Heavy Weapons to choose from. Then there's your role - which determines what sort of extra support you can call in. As you add kills in the course of a match, you earn extra '"role points," unlocking better and better weapons - so if you keep respawning as the same character type, you'll level up faster than if you keep on switching styles.
The best all-purpose pick, letting you drop an array of gun emplacements. Useful when you really don't want to let go of that flag you've just spent ten minutes capturing.
Probably the most "fun" class, which lets you drop anything from precision strikes and gunship fire to cluster bombs.. and, eventually, a tactical nuclear strike.
The ones with the toys - from a simple remote drone, working up to a miniature tank. Of course, while you're fiddling about with pitch and yaw, you're vulnerable from behind.
Counter measures players get warned about incoming missiles or drones, and can set off chaff or EMP blasts to nullify them. You'll want one of these boys if you're in a tank or chopper.