Defenders are there to defend the MCOMS. Normally the defenders don't have vehicles and you're there with just your soldier and the equipment that they carry. Defending is one of the hardest things to do in rush because you need constant coverage on the MCOMS and you need an equal balance to handle the oncoming ATVs, tanks, and UAVs. Defenders, well, defend the MCOM and would disarm it before it explodes. You disarm it by pressing e until it stopped beeping.
Attackers is one of my favorites to play. You get tanks, UAVs, ATVs, and sometimes helicopters to advance your way through the map to destroy all the MCOMS. You can either destroy them by shooting RPGs or UAV rockets at them, or you can arm it but holding e until it beeps. When it starts beeping, your basically defending your attack on the MCOM. You really have to get your crap together and stay alive, as you only have 100 (I think) tickets or you lose. If you have a well-playing team, you should sweep through most MCOMs.
It's a lot different from conquest, and honestly can help you get a feel for how you can attack and defend flags within conquest. I was a rush player before I started to enjoy conquest. They both have their goods and their bads, but after you get the feel for rush, it's a great game mode to play. It involves a lot of teamwork and a lot of strategizing.
