What is each classes role / responsibilities in a raid?

[NPC]

You mean paladins?

Really you need a couple max level alts if your looking to raid, you can still main any class, but should keep either chanter, cleric or warrior(ogre) alts.
Only the best decked BIS warriors are considered for raid MA, an that's who has the most fun. Other than that, any other tank class can MA for the alt groups.

Raid responsibilities-
Chanter - Mez, slow, tash, Buff
Cleric - Complete Heal, Buff
Warrior - Tank

Everyone else is off tank, buff, heal or DPS, that's it. The main raid group does most the work an everyone else is waiting and assisting. So it can get really boring if you're not the main raid group(s), and those are usually reserved for BIS characters an specific classes depending on the raid. Pullers can vary, ranger, bard, monk, SK, but with Mezz an solid competent groups, that doesn't really matter as much either.
The entire strategy for 99% of all raids = CH chains and Ogre warriors that can hold agro makes the world go round. Chanters/shaman slow an haste.
Which is why you want an alt of with chanter, cleric, or warrior, they are the only essential classes.