The .swc file is the component compilation of the components.With this file included in the other projects of the lib folder [using Flex Builder IDE] we can use those compiled components.After include it in the lib foler of the project ,the components are added to the Custom folder in the Components view.
To do this we need to create a Library Project in Flex Builder, and create components there with our properties and functionality and the relating .swc file is created in the bin folder in that project which is the compilation of the components we define in the Library Project,we can define and create as many components in the Lib Project and the corresponding .swc file is updated i.e the compilation of the components are written in it.
So, when your are done with defining the components in Lib Project add the .swc file in the lib folder of the required Flex Project where you would like to use this custom components.
Now drag the CustomComponents from the Custom folder in the components view on the design view and the required functionality is written in them and we can add other funtionality.
This is with the Flex Builder, if not using the Builder then define the components in a folder and compile the components in that folder it will generate the .swc file.
For compilation use the command promt with the commands like