Original Equipment Manufacturer.
is what you get "bundled in" free
when you buy a computer. It might include Windows, some complete older programs, "taster" versions of new programs, etc.
OEM stuff is unsupported and comes without
manuals or any kind
of help. It's a free
gift, take it or leave it.
Buying second-hand OEM software
can get you a real bargain, as long as you know
enough to get by without
the manual or support.