Acronym for Original Equipment Manufacturer.
OEM software is software sold under special agreement for the purpose of bundling with hardware. Computer vendors are typically OEMs. There is also a market for OEM software sold unbundled at very low prices, which is usually in violation of the agreement the OEM has with the software company.
OEM software is usually not accompanied by regular packaging, such as a box, and may come without manuals.
OEM hardware is similar to OEM software - sold to computer makers at lower prices, without the packaging, etc.