What is an external and internal command?
In MS-DOS there are two ways commands are executed. An Internal command, which is a command embedded into the command.com file, and an external command, which is not embedded into command.com and therefore requires a separate file to be used.
For example, if your computer does not have the fdisk.exe file and you try using the fdisk command, you would receive an error "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 any other files to run.
Each of the commands listed on the Computer Hope's MS-DOS page lists what commands are external and what are internal on each of the command pages.