The World of Warcraft's Holy Paladin is one of the three specializations of the Paladin class, the others being Retribution and Protection. As Holy you are pretty much a pure healer, so foget about ever seeing the top of a damage chart again. Reaching the top of the "most healing done" chart might be a whole 'nother story.
Before Cataclysm the Holy Paladin spec used mana, just like any other healer or spell casting class. With the arrival of the Cataclysm expansion all Paladins now have a "Holy Power" mechanic in addition to their mana use. With Holy power the Paladin uses various abilities to build up to three points of Holy Power and then another ability "unleashes" those points to power some ability. With a Holy Paladin, of course, that ability will generally be aimed at healing someone.
Also coming with Cataclysm was a complete overhaul of the talent system. Players can no longer mix and match points from all three talent trees, but have to stick with one tree (one "specialization") until they have spent 31 points in that spec. Players will eventually have a total of 41 points to spend at level 85, which means that more more than 10 points will every be spend outside the Holy tree. With every specialization for every class comes a set of unique abilities. For the Holy Paladin these abilities are:
Other flavors of Paladin will not be able to get these particular abilites and you will not be able to use theirs. This means that every specialization of every class will have certain unique abilities and, with the Holy Power game mechanic, Holy Paladins will have a different feel from any other healer. They will have to manage both their mana and their Holy power at any given time.
When building your Holy Paladin you have to decide whether you will be a pure healer or if you will be doing some damage as well. Protection and Retribution Paladins are both melee types which gives them a more limited view of the fight, since they will always be in the middle of it. You will be some distance away, healing, which gives you a different picture of the fight. In PvP, of course, since you ARE the healer, you can often count on the fight being in your face as the opposition tried to bring you down. Naturally your pocket Warrior will have something to say about that.
If you will be a pure healer, for example, you won't be taking talents which only increase damaging attacks. Whether you're in PvP or PvE combat makes a difference, too. For example, you will eventually have the option to take "Crusade" from the Retribution tree. It looks like a straight up damage boost, but note that if you have three points in that talent and you kill one of a Shadowpriest's "Shadowy Apparations," very easy to kill, you get a 300% increase to your next "Holy Light" spell. Of course, in PvE there's plenty of stuff that your team will be killing. So that talent is a bit more than it seems at first glance.
While you will always be primarily a healer, depending on your exact Holy Paladin Build you might also be able to do some damage. Leveling solo will be tedious, but sometimes it's nice to be able to whack something instead of just healing your team. Of course, having an entire team counting on you to keep them healthy also has its virtues.
A well played Holy Paladin can be very hard to kill in PvP and they're also valued as primary healers in instances and raids.