Flash is a proprietary graphics and animation format owned by Macromedia. The file format was opened a while ago, with Macromedia also offering large amounts of source code to developers who wanted to develop Flash-compatible programs - although getting the source code required accepting the Macromedia licence agreement.
Flash movies themselves are vector-based which means that they remember the shapes you draw, the fills and gradients, etc, by way of mathematics - this means the movies can be resized and scaled with almost no loss of quality. Note that you can import bitmap pictures into Flash, and these are the only thing that don't resize well.
The Flash format supports streaming animation, sound and music, morphing, and also HTML-like forms capable of submitting and receiving data.