PCI/PCI-Express (aka PCI-E) slots let you install expansion cards.
Expansion cards give your computer additional capabilities. Some very common expansion cards were sound cards, 56k modems, and Ethernet adapters. You don't have to populate your PCI/PCI-E slots, but it is an option if you need to extend your hardware.
Since the introduction of the PC in the early 1980's, more and more capability has been included on the motherboard. Most motherboards today include sound capability and (sometimes multiple) Ethernet ports.
The card must match the slot, e.g. if you have PCI-E slots, you need to buy a PCI-E card. Newer slots support faster hardware. PCI-E is the newest and fastest. PCI is still very common and included in motherboards. Older slot types include EISA (black) and AGP (brown - for graphics cards only).
No one makes RAM for PCI/PCI-E slots, RAM has had its own dedicated slim slot type on motherboards for about 20 years now (in the ISA days you could buy memory boards that plugged into the ISA slots). Hard drive "cards" were a short experiment in the early 80's and no longer are made. Hard drive controller cards are common though - these are just cards that sport additional IDE or SATA ports and let you attach more drives - but you need separate power lines for the drives off your power supply and you're responsible for having a case that lets you put them somewhere.
Both PCI and PCI-E have "mini" versions for laptop use.
Thanks for the answers ultrasawblade and journeyman. I realise this question may seem a 'stupid' question for someone building a server but this is my first ever build and I've never had to worry about what PCI and the variants are until now. There are plenty of articles that talk about each specification of PCI but none that I could find gave a good overview of the technology and what they can be used for. – Dean – 2012-08-01T21:52:14.183