Essentials is not a different game from 4e D&D. It is simply a new line of products that should form the new path into D&D for beginning players.
The first boxed set has just 4 classes and 4 races and three (or two) levels of play. Presumably each class will just have one or two builds with predetermined choices (but this is just speculation).
Other products in the line are a boxed set for DMs, a set of monsters, a Compendium with the full rules, two books with full options for several classes and races (this stuff should be mostly new), 3 set of tiles and a dice set.