The usual differences is packaging, warranty, & software. Sometimes, there are hardware (cutdown) differences.
MOST of the time, the OEM and retail are the same, except for software, packaging (white box), and warranty.
e.g. CPU's retail with 3 years warranty, and nice packaging and a fan. OEM has oem warranty (none, to 15, to 30 day warranty) and no fan.
video cards my vary by software (games) or drivers (no driver cd). Sometimes, usually ATI, differ in retail and OEM by cmemory and lock speeds also.