[Mark McLeod]

I believe you're thinking of mcp.
.mcp is the Project information and is what you select in MPLAB to open a project. It specifies the files in your project, build options, where the compiler to use is located, that kind of thing. It's the only really important MPLAB file you need to preserve in a project directory.

.mcw is the Workspace information file. It remembers what windows you left open the last time you closed MPLAB, configuration settings such as the default window size, whether to save modified files before building, that kind of thing. If it's missing MPLAB will just create a new one.

mcs are build state files. They come and go.