Druid: Main healers of Hibernia; also gain specialized stat buffs, pets, DOT's. If you want to play a pure healing class, this is the closest you'll find in Hib.
Bard: Secondary healers, and Hibernia's Primary CC'ers. Gains group endurance-, power-, and health-regen songs, speed, etc. Also gains a variety of cast, insta-case, AOE, etc. mezzes. These guys are essentially a group-only class. Luckily, they are one of the most sought-after by groups.
Warden: Since you aren't as clear on them, I'll be less succinct: Think of a Warden as a druid/tank hybrid, but with a twist. Wardens are still on the Naturalist damage table, so they'll never do as much damage as a Hero, Blademaster, or a Champ. That said, they still get to wear Scale (Hib's best armour) and soak up damage, have self haste buffs, and a number of chants. Chief among these is the Pulsing Bladeturn (Or PBT) chant line. Every 10, 8, or 6 seconds (depending on level), the chant pulses a Bladeturn on everybody in the group. This absorbs one melee hit and then "pops", giving Bladeturns the nickname "bubbles". In PVE this is great, since everyone in the group can take an extra damage-free blow every few seconds. In RvR, there are ways around it (multiple tanks concentrate on one opponent, double-attacks only have 1 hit deflected etc.), so it's not as good at that level. Add to this the same buff, healing, and res spells Bards get, and you get a character that can fill several holes in a group. Need a tank? Need a healer? How about both?
Now the downside: Like many hybrids, Wardens' specialization isn't in any one area, so they have a hard time bringing something "unique" to a group. Sure they can absorb damage, but they can't do as much as a Hero. Sure they can heal, but not as well as a Druid. Now, this doesn't come into play so much until the highest of levels, when getting the perfect group is thought to be more important. Personally, I've never had a problem getting a group, but my Warden's not to 50th yet. : )
--Galilipip