Clean Solution Deletes all compiled files (all dll’s and exe’s ).
Build Solution Compiles code files (dll and exe) that have changed.
Rebuild Solution Deletes all compiled files and Compiles them again regardless of whether or not the code has changed.
Build means compiling and linking only the source files that have changed since the last build. The build is the normal thing to do and is faster.
Rebuild means compiling and linking all source files regardless of whether they changed or not. Sometimes the versions of project target components can get out of sync and rebuilding is necessary to make the build successful.