Kind of the Jack-of-all-trades. You can tank, heal, damage. Just not as good as Guardian, Cleric, etc. You can help tank when main tank needs relieved, you can quick heal in emergencies if Cleric gets too busy, etc.
My normal play group is a pally , me (chanty), a druid and a shammy, in our group the pally is our main tank and he does just great . When grouped with a Guardian the pally is great TM until stuff can be mezzed , the pally is awesome at getting agro off a caster asap. They really can turn the tide of a battle IMO , and I love grouping with them .