From Gnash Project Wiki
ActionScript (or "AS") is the scripting language for Flash applications. It gets compiled into bytecode by an ActionScript compiler. The bytecode is a subset of the SWF format, in particular a compiled ActionScript ends up as a DOACTION SWF tag type.
There are 3 versions of ActionScript: AS1, AS2, AS3.
A player doesn't case about AS syntax, as it will only get an SWF to interpret. From a player's point of view, new AS versions can only introduce new action types. Action types are opcodes sequentially found into a DOACTION tag.
ActionScript code can act on scriptable characters of a movie.