The .obj file, what is it?

[Adam D. Ruppe via Digitalmars-d-learn]

Basically, .obj is a temporary file the compiler uses to store 
its half-finished work on the way to producing the exe.

Once you have the exe, the obj is no longer necessary, but 
keeping them around can sometimes speed up recompiles by reusing 
the left over work from last time. (Not so much in D though, this 
more applies to C.)