What are object files in c?

Object files are intermediate files generated when an application is constructed from source code. In a classic compile-assemble-link toolchain, the compiler translates (C language) source code into assembly language output. The assembler translates this intermediate assembly source into a binary form known as object code. Files containing object code often have a .o or .obj file name extension and are generally called object files. The linker takes the object files and combines them into the complete executable.

While many modern tools hide the individual steps, or even integrate compilation and assembly into one, most (all?) compilers generate object files.