All about incarnates: A player help guide. I recommend starting here.
http://boards.cityofheroes.com/showthread.php?t=257011
Edit: Nice short answer on -what- incarnate powers are and do:
They are part of "The End Game" for level 50s that people have been clamoring for a long time. The designers aren't going to add more levels to the game, unlike WoW. So this is their solution. There are several new Trials/Task Forces (Apex, Tin Mage, Lambda, BAF) that either require unlocking the Alpha slot or your character won't be as effective until you do. Yes - you can run Apex and Tin Mage without having unlocked Incarnate powers.... good luck...
But basically Incarnate powers are all the new stuff that can be unlocked once you're level 50. There's a storyline involved which explains where they come from and how/why they exist. They're very thoroughly thought out.
http://boards.cityofheroes.com/showthread.php?t=257011
Edit: Nice short answer on -what- incarnate powers are and do:
They are part of "The End Game" for level 50s that people have been clamoring for a long time. The designers aren't going to add more levels to the game, unlike WoW. So this is their solution. There are several new Trials/Task Forces (Apex, Tin Mage, Lambda, BAF) that either require unlocking the Alpha slot or your character won't be as effective until you do. Yes - you can run Apex and Tin Mage without having unlocked Incarnate powers.... good luck...
But basically Incarnate powers are all the new stuff that can be unlocked once you're level 50. There's a storyline involved which explains where they come from and how/why they exist. They're very thoroughly thought out.