If you have a newish version of DOS there may be a help program provided
with it which will contain a list of commands.
MS-DOS, versions 6.0 and 6.2X, come with detailed help on all the DOS commands and
files etc. (the Windows 9X CD also has it). You just type help.
DR-DOS (not sure which versions) has dosbook, which provides
the same help topics as MS-DOS help and many more, in fact its much more
useful (but only for DS-DOS).
In addition to these you may find the guide to DOS commands
I've written useful: