The Galaxy Editor is the collection of mapping and modding tools that will come with Starcraft II. It is the successor to StarEdit from the original StarCraft. Blizzard has claimed that nearly every feature from WarCraft III's World Editor is present and improved in Galaxy Editor.
All units from the singleplayer and multiplayer game will be available for use in the map editor. Units that have been cut from the game completely will not be in the editor. It is not currently known if scrapped models and revisions of units still present will be available (e.g. the Nullifier's model or a Phoenix with Overload).
The Galaxy editor will offer two ways of creating custom scripting for your map. A trigger editor will be available for novice and intermediate users. This will be a user friendly way for mappers to create custom scenarios. For the Advanced users, Blizzard has created a C-like language called Galaxy that can be used to code custom scenarios. This language will not be Object Oriented.
Designers who use the galaxy editor will be able to craft custom cut-scenes and "briefing rooms" for their campaigns directly in the editor. Briefing rooms, cut-scenes and regular gameplay all use the same game engine to be rendered in the game.
The game-play of Starcraft will be highly customizable in the editor for anyone willing to the scripting work that will be required. Although Starcraft is a real time strategy game, you will be able to create nearly limitless other game types. Examples of maps have been seen showcasing third person shooter ( similar to Splinter Cell or Tomb Raider ) and top down arcade shooter games like Raptor.
There has also been a lot of talk confirming support for Aeon of Strafe / Defence of the Ancients (DotA) type game-play. Many hero related functions will be available including character leveling, items, and character statistics. Units can have an unlimited number of statistics, however there is only room for three statistic to be displayed on the UI. DotA is any extremely popular game type on battle.net in Warcraft III.