A description is at http://scratch.mit.edu.
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Scratch is a building block style GraphicalUserInterface ProgrammingLanguage created by MIT Labs.

This programming language is heavily GUI based, with three still supported versions at time of writing.
* Scratch 1.4 of 30-Jun-09 - Java-based Squeak - Local application
* Scratch 2.0 - Flash object - Online Application, embedded in HTML with minimal HTML script
* Scratch 2 Beta Offline Editor v404 - Adobe Air, possibly running application (Scratch.exe) embedding Scratch.swf - Local Application with Glitches: see GlitchesInScratch
It has chunks of code (scripts), which are activated as shown by their headings, e.g.
* When <Green''''''Flag''''''Icon> Clicked
* When I receive <Broadcast>
* When <key''''''Name>  key pressed

There can be several scripts in each Sprite, and no sprite can tell another to do anything. (That is the purpose of Broadcasts)
Each Spite can have as many costumes as the computer can cope with (there probably is a limit, but it'll be hard and long to get to, and it will differ between versions), which can all be different sizes.

TBC --SimonMould
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The commands the language use are in the page ScratchLanguageCommands. --SimonMould