What's an object file in C?
An object file is the real output from the compilation phase. It's mostly machine code, but has info that allows a linker to see what symbols are in it as well as symbols it requires in order to work. (For reference, "symbols" are basically names of global objects, functions, etc.)
A linker takes all these object files and combines them to form one executable (assuming that it can, ie: that there aren't any duplicate or undefined symbols). A lot of compilers will do this for you (read: they run the linker on their own) if you don't tell them to "just compile" using command-line options. (-c is a common "just compile; don't link" option.)
An Object file is the compiled file itself. There is no difference between the two.
An executable file is formed by linking the Object files.
Object file contains low level instructions which can be understood by the CPU. That is why it is also called machine code.
This low level machine code is the binary representation of the instructions which you can also write directly using assembly language and then process the assembly language code (represented in English) into machine language (represented in Hex) using an assembler.
Here's a typical high level flow for this process for code in High Level Language such as C
--> goes through pre-processor
--> to give optimized code, still in C
--> goes through compiler
--> to give assembly code
--> goes through an assembler
--> to give code in machine language which is stored in OBJECT FILES
--> goes through Linker
--> to get an executable file.
This flow can have some variations for example most compilers can directly generate the machine language code, without going through an assembler. Similarly, they can do the pre-processing for you. Still, it is nice to break up the constituents for a better understanding.