An SWC file is a package of precompiled Flash symbols and ActionScript code that allows a Flash or Flex developer to distribute classes and assets, or to avoid recompiling symbols and code that will not change. SWC files can be generated by the Adobe Flash Professional authoring tool, and by Flash Builder (or its companion compiler MXMLC). They are sometimes referred to as class libraries and cannot be directly executed by the Flash Player.
SWC code libraries are typically included in an ActionScript 3 project, and compiled into the final SWF file alongside the developer's program, by the ActionScript 3 compiler. Only the classes that have been used by the project (and their dependencies) are transferred into the resulting SWF file.