New: Just like it sounds. A brand-new, unused, unopened item in its original packaging, with all original packaging materials included. Original protective wrapping, if any, is intact. Original manufacturer's warranty, if any, still applies, with warranty details included in the listing comments.
Refurbished: A refurbished product has been professionally restored to working order. Typically this means that the product has been inspected, cleaned, and repaired to meet manufacturer specifications. The item may or may not be in its original packaging. The manufacturer's or refurbisher's warranty must apply and should be included in the listing comments. Refurbished items are sometimes referred to as "remanufactured."
OEM: OEM means "Original Equipment Manufacturer". OEM products are usually aimed at computer assemblers or "system builders", and designed be installed or used with a newly built PC or a PC being upgraded. Typical OEM products include bare hard drives, motherboards, sound cards, software and keyboards/mice. If you buy a product as "OEM", it will usually be cheaper that the RETAIL equivalent because, you won't get retail packaging (a fancy box), a user manual, software drivers or installation instructions.
Returns: An item that has been used previously. The item may have some signs of cosmetic wear, but is fully operational and functions as intended. This item may be a floor model or store return that has been used.
Shelf Pulls: Shelf pulls are typically overstocked goods that have been displayed for sale in a store or online but have never been purchased by a customer. Shelf pulls are typically considered excess inventory from store shelves. In the vast majority of cases, shelf pulls require much less work than customer returns in terms of testing and repairing, but you should expect to have some checking work to do as well as cleaning off potentially dusty items and removing price tags (if necessary) before reselling the items.