For our purposes here, a "project" is all the "stuff" that is associated with the making of a particular PCB. The main files are as follows, but there will be others, too… backups, caches, etc.
- Schematic (.sch file, etc)
- Netlist (.net file)
- schematic symbol to Footprint assignments (.net files, and, before vers 4.0.4 or earlier, .cmp files)
- PCB artwork (.brd file (legacy, at 4.0.4))
- The libraries
…. and, of course the file with the ".pro" extension, which is the "one ring to rule them all."
http://kicad-pcb.org offer an excellent page on the files that make up a KiCad project.
(This entry may need improvement. If you particularly want THIS expanded, write and cite "Page co1proj")