At level 20:
Paladins can do the most damage and have huge heals along with some mobility, a stun and the Divine Shield. Also they can remove debuffs. (mail armor+ shield)
Exceptional defensive abilities, but they can't control a group so much. They just have power.
Druids have stealth, great HoTs, Entagling roots (which is ranged), DoT for stealthers and they can avoid snares and polymorph. (leather armor)
Learn when to run with them and they will serve you well. They can't control a group either, but they can disorganize it sometimes.
Priests can make a group better with 1k shields, can break other groups with their fear abilities, have DoT for stealthers, but they have no speed buff. (cloth armor)
Really helpful in battles between big groups, but quite vulnerable if many attackers on them. Use the cc abilities wisely.
Shamans can be more offensive against healers with Purge and Wind Shear, they have Ghost Wolf, HoT and the usual healing abilities, DoT for stealthers and some extra tools that
might make the difference. They can remove debuffs too. (leather armor+ shield)
Not really defensive and since they can't cc, you should focus on supporting your attackers.
I hope the information was right and helpful.