A broadcast is a message that is sent through the Scratch program, activating receiving scripts. Broadcasts are sent with the blocks Broadcast () and Broadcast () And Wait, and are received by the block When I Receive ().
(note: little known fact: you can simulate a start button press by just puting in "scratch-startclicked" in the broadcast block)
This block, a Hat Block, will stay inactive until it receives the specified broadcast. Once it has been received, the script goes into action and ends once it has finished - but it can be started more than once.
I Receive ()Edit
This is a non-existant boolean block that many Scratchers want. This block would most likely belong in the Sensing category.