The .obj file, what is it?

[I Lindström via Digitalmars-d-learn]

So, a question from a beginner. What is the .obj file that 
appears after the source is compiled into the executable? I can't 
find a good explanation on the Net for it. I take it the file has 
to accompany the executable for the program to function since the 
online explanations I've found say it contains instructions and 
is related to memory management? It's been bugging me from the 
start.