COMMAND.COM: supplies the A> prompt and accepts commands from terminal or from a simple batch facility
Some commands are built-in to COMMAND.COM - e.g. COPY, DELETE, DIR, TYPE, etc. (These are called internal commands.)
Other commands are implemented by running a utility program included with MS-DOS - e.g. DISKCOPY, FORMAT, PRINT, etc. (These are called external commands).
Application programs can be invoked by typing their name, provided an executable file name.COM or name.EXE resides on the disk.