OEM generally means "original equipment manufacturer."
Almost all PC component and software venders make two versions of the same product based on a difference in the distribution channels.
The first 'version type' is retail boxed products. In the case of software products such as Microsoft Windows, Symantec Norton Antivirus and others, the retail versions include the CD, manual and any other information applicable to the product. These retail versions are packaged in a sealed box much like you would see at a retail store.
The second 'version type' is OEM products. Generally, OEM products are sold mainly to system builders, such as Compaq and Dell. These bulk-pack products do not have the retail packaging materials and are often produced in much higher volume, often resulting in lower prices for the end consumer. The OEM products are of the same quality and often carry the same manufacture warranty as their retail counterparts. OEM software products may or may not include supporting materials such as manuals. OEM hardware products generally do not include cables, mounting hardware, software utilities etc.
Note: Because OEM products differ with regards to warranty and included materials, it is always a good practice to check with the reseller before purchasing.