A bard is a very flexible and fun class to play.
They are great pullers when they use their aggro range reducing songs and at higher levels those songs and fading memories.
They can kite very effectively, if you do not mind a loooong fight, once you get your dot songs.
They make great downtime health/mana regen assistants.
They provide very nice group haste/damage shield/proc songs.
With all of your defensive aa's in place you can make an adequate tank for normal group mobs. You are not a warrior, but you can take some hits and keep on going.
Bards are great explorers with their song of travel.
You can cure disease, poison, curse. You can give a group levitation or invisibility. Bards can do all sorts of stuff. You can even pick locks, track, safe fall, disarm traps, mez mobs,charm mobs, dispell, blah, blah, blah.
What is the catch? Your melee DPS is NOT that of the other melee classes. Your dots will always be somewhat small (critical affliction is your friend), your tanking ability is not that of a true tank, you never get backstab. They start very weak, and don't get 'cool' until 60+ (my opinion, because of more multi-function songs = stronger combined twisting).
The short answer is they can do a little bit of a lot of things at the expense of not being exceptional in any category other than flexibility.
If you do play you will want to know about this command...
/melody # # # #
This will allow you to twist four songs effortlessly where the # is the number of the song according to how you have them memmed on your spell jem bar. 1-8, with one being the top number.