Build a laptop and you don't even know the components inside a system? Good one ...
As far as internal componenets go ... you've got ..
A power supply - gives power to the entire system. It's a DC converter basically. Almost like a transformer that you see on power poles.
Then there's a motherboard. It's what your internal components connect to and is the core of a system.
Then memory. It's called RAM (aka RANDOM access memory) .. and it's very important as well. There are many types of RAM availible at different speeds, voltages, timings etc as well so you need some help deciding on your needs.
Then a CPU (or processor) .. which is achip that basically gives your system its ability to compute. There are also many, many types of CPU's out there at different speeds and such as well. So you will again need to decide on what socket you need, the power you want, and things like that.
Then there's your accesories like a video card, a LAN card, etc. Then of course, your DVD drive, your CD drive, floppy drive, or whatever else you want to modify your system with. Oh, and of course you're going to need a case to put everything in. Motherboards come in many sizes and designs and your case will have to be large or small enough for the motherboard you decide to buy. Whether it be Mini ITX or a full ATX case, it's your choise.
Anyways sorry but I don't feel like giving a thorough how-to on how to build an entire system right now. It's complicated to describe and will take a long time. And I still probably wouldn't be able to tell you everything you'll need to know. Honestly, you need someone to teach you how to build a system and what all the parts are IN PERSON or else you will probably end up buying a bunch of parts and promptly fry them all.
Yeah, this isn't exactly a detailed description on how to build a system, but these are your main components of one.