WotC has stated the following in the Thief preview:
- The Essentials consist of 10 key products that will form the baseline experience for the roleplaying game moving forward.
- The Essentials are NOT a new game or a new edition.
- The Essentials are NOT replacing the Player's Handbook, Dungeon Master's Guide, or Monster Manual.
- The Essentials are providing a better framework and starting point for new players while also providing new options for existing players to add to their games.
- The Essentials products work with all other Dungeons & Dragons roleplaying game products, from Player’s Handbook to new products releasing in 2011 and beyond—just like all other D&D products we release.
The specific composition of the 10 "Essentials" are:
1. Dice
Three sets of Dungeon Tiles for the following environments:
2. Dungeon
3. City
4. Wilderness
Three boxed sets:
5. Introductory Red Box starter set (includes game rules, dice, maps, tokens, a solo adventure that guides a new player through the character creation process, and an adventure that takes characters from 1st to 2nd level)
6. Dungeon Master's Kit (includes game rules, advice, adventures, maps, tokens, and a DM Screen)
7. Monster Vault (includes monsters, monster tokens and an adventure)
Three books:
8. Rules Compendium (the essential rules required to take a campaign from 1st to 30th level)
9. Heroes of the Fallen Lands (contains rules for the cleric, fighter, rogue, and wizard classes, and the dwarf, eladrin, elf, halfling, and human races)
10. Heroes of the Forgotten Kingdoms (contains rules for the druid, paladin, ranger, and warlock classes and the dragonborn, drow, half-elf, half-orc, and tiefling races)
As for your side question, it is very likely (IMO), but not confirmed, that the new builds will be in the Character Builder.
WotC has also emphasised the following (quoted from
Countdown to Essentials):
"[T]he Essentials products matter only as much as you want them to. We very carefully designed the new classes and added more options to the races in such a way that existing characters remain unchanged. Aside from the rules updates introduced over the past few months, of which the relevant pieces are included in the Rules Compendium, little (if anything) on your character sheet has changed. The only real changes rest in wizard encounter spells (they have miss effects now), and those changes are almost entirely additive in nature. Your burning hands spell is the same spell as before, except now it deals half damage on a miss.
This point bears repeating—Aside from rules updates and changes to one category of wizard spells, the character you are playing today does not change in any major way. It was crucial to us that someone playing a dwarf fighter today didn't need to rebuild that character once the Essentials products were released."
Some have interpreted the bit about adding more options to the races to mean that some (if not all) of the Essentials races may be able to apply their racial bonuses to different ability scores (e.g. instead of having a fixed +2 Dex +2 Cha, a halfling may have +2 Dex +2 Con or Cha), or may be able to choose different racial abilities. However, the PH1 version will always remain a valid choice even if it is no longer the only possible choice.
