-c
Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o. Unrecognized input
files, not requiring compilation or assembly, are ignored.
-o file
Place output in file file. This applies regardless to whatever sort of
output is being produced, whether it be an executable file, an object
file, an assembler file or preprocessed C code. If -o is not
specified, the default is to put an executable file in a.out, the
object file for source.suffix in source.o, its assembler file in
source.s, a precompiled header file in source.suffix.gch, and all
preprocessed C source on standard output.
More can be found in GCC Manual Page
