Component classes can take the form of MXML, ActionScript files, or as SWC files. In MXML or ActionScript files, the components are not compiled but reside in a directory structure that is part of your compiler's source path. SWC files are described in SWC files.
Component libraries are not dynamically linked unless they are used in a Runtime Shared Library (RSL). Component classes are statically linked at compile time, which means that they must be in the compiler's source path. For information about creating and using custom component classes, see Creating and Extending Adobe Flex 3 Components.