>>327 did a pretty good job of summarizing it.
There are alot of standards, types and derivatives of most computer parts (SATA vs. ATA; DIMM vs. 184pinDDR2SDRAM vs. 240pin DDR2SDRAM; ect). It is also quite unrealistic to expect to get 'the best one on the market' unless you have a shitload of discretionary money, and assuming you did, it would be best to pay an IT guy to do it for you. If not, buy or P2P a "Computer Hardware for Dummys" type of book.
What exactly do you want to do? If you want to build a computer, it's best to buy a junker in the classifieds in your local paper, and upgrade it, plus it's alot cheaper.