OEM = original equipment manufacturer... Idea of OEM is items to be sold not to end user but people who assemble computers. Strictly speaking OEM products are only supposed to be sold as part of a whole computer, logitech often try and enforce this, most manufactueres dont bother, probably because its not worth the hassle and also OEM means an easy way to price discriminate - sell the same product to two "close" markets at different prices.
So whats the actual difference? OEM also often does not come with a full warranty - in some cases you'll only have a 30 day window to return an OEM item, compared to 1 to 3 years for the retail product. This varies widely from manufacturer to manufacturer. A significant difference in warranty is the NORM between retail & OEM versions - this is a significant factor in why OEM versions are often so much cheaper.
Second important difference is the actual product maybe be subtly - but significantly - different, an example already given is videocards with slower memory/gpu clocked speeds, or the same clock speeds but lower rating so it doesnt overclock aswell. Also accessories may not be given, such as maybe the retail version of a example HDD comes with the IDE cable, and the OEM might come as the drive only. Manuals are usually on CD for OEM, but in paper for retail - although even retail versions often seem to have manuals only on cd now. Almost always the OEM packaging will be plain cardboard - sometimes even repacked by your retailer as theyve arrived in boxes of 100 etc
With regard to your CDRW, I'd guess the only difference is likely to be the packaging, the picture shows the IDE & sound cable, a printed manual plus CD with Nero express... If youre going to buy a OEM version (i ususally do where possible) it is worth checking the specs are not lesser in something important, often though the only real difference is the warranty, and sometimes not even then.