Processor -- Intel and AMD are the two major ones.
but which is better?
Depends on what you want to do. For a gaming PC, go with Athlon. If its a workstation, go with Intel.
Motherboard -- needs to be compatible with your processor.
OK, processors are mostly all different. Each one attatches to the motherboard differently. For example the Athlon XP 2600+. To put that on a motherboard, it must be a "socket A" compatible motherboard. When looking for a motherboard you look for which socket it is, for athlon xp it would be socket A. Research whatever processor you're getting to find out what socket it is(its not hard).
Asus/Abit is most likely the most trusted brand names. Definently go with one of them(ASRock is good too, its a subcompany of Asus)
RAM -- DDR is the RAM of choice now days because of it affordability and stability. Has to be compatible with your Motherboard..
Well when you read a motherboard specs you need to see what type of memory it supports. It's pretty basic now, most are supporting up to PC3200 RAM. You need to find out how many DIMMs(slots) of RAM there are, then you can
get that many sticks of RAM(getting just 1 stick is fine.).
Most brand-name RAM is compatible with all motherboards if it is PC3200 or lower, but some gerneric RAM types will ask for specific things that most motherboards do not have(like x4DRAM, 4 layer generic modules). Some RAM may even require a specific chipset.
my suggestion is 512mb PC3200 DDR400. Its cheap... and its good.
Case -- Used to hold the above components. They come in several shapes and colors. Needs to be compatible with your Motherboard, ie. ATX, Mico-ATX.....
Video Card -- The thing that makes all of the pretty colors on the screen a.k.a The monitor (thanks to str8lazy).
I've tried both ATI and nVidia.. and my opinion is nVidia > ATI.
Hard Drive -- Where software and data is stored. Really important if you want the PC to be functional. (thanks to sithspawn)
Yes theres a lot of different kinds..
Theres normal ATA hard drives(I suggest you go with this).. the ones connected by the normal ribbon cables(or rounded cables).
Theres SATA hard drives.. which I don't know about, but they are newer and much better.
the RPM of the hard drive is a big factor, definently get a minimum of 7200RPM. The higher the RPM, the more expensive...
go with Western Digital, I've never had any hassle with them and it's lasted for more than a year and a half now.
Oh, and you should also try to get 8mb cache on your hard drive... and somebody else can explain to him what EIDE/SCSI is im lazy and just wanna end this post..