What is an external and internal command?
In MS-DOS, there are two ways commands are executed: internally and externally. An internal command is embedded into the command.com file, and an external command, which is not and requires a separate file to operate.
For example, if your computer does not have the fdisk.exe file and you try using the fdisk command, you will receive a "Bad command or file name" error message. Fdisk is an external command that only works if fdisk.exe, or in some cases, fdisk.com, is present.
However, as long as MS-DOS is running on your computer internal commands, such as the cd command, will always be available and does not require other files to run. Each of the commands listed on the MS-DOS help page denote what commands are external and internal.