Forgot to add something about heroic instances.
You access them by buying a so called "heroic key" from different factions. You will need (at least) Honored reputation with them and be level 70

Those are:
Sha'Tar - for Tempest Keep instances (Mechanar, Botanica, Arcatraz)
Lower City - for Auchindoun instances (Shadow Labs, Mana Tombs, Sethekk Halls, Auchenai Crypts)
Thrallmar - for Hellfire Citadel (Ramparts, Blood Furnace, Shattered Halls)
Cenarion Expedition - for Coilfang Reservoir instances (Underbog, Steamvault, Slave Pens)
Keepers of Time - for Caverns of Time instances (Old Hillsbrad Foothills, Black Morass)
How do you get a key? Once you hit honored go to the faction quartermaster and you can purchase it. Note that all members of the party need to have the key in order to access heroic mode.
In heroic, all mobs are 70+, they hit a lot harder and bosses have additional abilities. Therefore, you need some good gear to be able to tackle them, but the rewards are also nicer (epics drop here, as well as some epic gems and Primal Nethers, used to craft special epic gear, as well as Badges of Justice)
Heroics are a good source for gear pre-raids and it's advisable to run a fair bunch of them before venturing into 10-man raiding, not only for gearing up but also for perfecting team play.
From the above instances, i find Shattered Halls the hardest to tackle (7-mob pull is so ftl), on par with Shadow Labs and Black Morass. On the other side, Ramparts and Blood Furnace are quite easy once you figure the quirks of the bosses so it's advisable to start tackling heroics by doing these.
Should any of you lads need more hints, let me know, I can also assist you class-wise for locks and mages, I'm afraid I don't know too many things about other classes tho :\