OEM is an acronym for "Original Equipment Manufacturer". Normally OEM software comes with brand new computers only.
The OEM software is absolutely identical to the software found in a full version retail box product. The difference is a printed manual and a box that come with a retail version. OEM software is usually distributed without a box and a printed manual included. OEM versions often come with only an electronic help manual which is built into the software. Purchasing OEM software one gets the full version of the program recorded on a CD-ROM disk and the serial number required for the installation of the program.
Since OEM software is typically sold to more advanced users, system builders and computer consultants there is no technical support provided by the software manufacturer. However, because of the substantial support savings to the manufacturer the OEM versions are offered for sale at a significantly lower price.
Retail versions of software are MUCH more expensive than OEM versions. Thus if you need the software, it's a great way to save tens or hundreds of dollars off of the retail, boxed version you would buy in the store.