If you are planning on buying a PC, I have to assume that you plan on using it. I believe that every computer user should have at least a basic understanding of how a PC works. When it comes to buying one, the requirements get tougher. Here I will give you an introduction to how computers work. The rest of the guide will build upon this knowledge.
If you feel that you already know enough, you can safely skip this part . This guide is presented in an easy to understand non-technical form, so most of the concepts here are gross generalizations. I hope the computer people reading this forgive me for that.

Computer 101
A computer is a fascinating device. It is made up of a bunch of electronic components that 'exchange' voltages and currents among themselves to give us something that apears to be intelligent and yet is very stupid. Let me elaborate.

The core of a computer system is a certain electronic component known as the CPU (Central Processing Unit). The CPU really understands only two things: the presense or absence of a certain voltage level on a certain wire. The people who design CPUs (Computer Engineers) connect the wires of the CPU in such a way that they can make it do interesting things - like adding, subtracting, multiplying, and dividing numbers and a few other tasks - depending on which wires have a voltage present (or absent) on them. Let us call these specially crafted presences and absences of voltage 'instructions' and let us call the numbers 'data'. The designers do not just stop there. They then connect the wires of the CPU to a certain set of devices collectively known as the 'Memory'. The memory can remember the presence and absence of voltages. By changing what the memory remembers, the designers are able to give different instructions and data to the CPU. Fascinating, huh? In reality, it is much more complex than this.

So what makes a computer special? Any 6th grader can perform addition, subtraction, multiplication, and division - why go through all the trouble of designing a CPU and a Memory just to do that? A 6th grader can probably perform one addition per 30 seconds while by changing the instructions rapidly, a CPU can perform millions of additions in just a second! The speed of the CPU is determined by another electronic component known as 'the clock' ("L'Clock" in French ). Let us just say the clock is the boss. The CPU performs the instructions when the clock tells it to. The faster the clock, the faster the CPU. Again, a question arises - how exactly do a bunch of additions and multiplications allow the computer to do complex things? Once the Computer Engineers finish their design they hand it over to Computer Scientists. Computer Scientists are excellent mathematicians. They can perform really amazing things by just using a sequence of basic mathematical operations. These sequences of operations are translated into instructions and data for the CPU and are fed into the memory. They are known as 'Software' or 'Programs'. When we, as users, are working with computers, we are usually working with these programs - consider Microsoft Word or the browser you are viewing this page with (hopefully not Internet Explorer ). Each of these programs have millions of instructions and data in them. The CPU executes each of these instructions in sequence. Bottom line: the computer is stupid - it just does what it is told; it is the person that is developing the software that is smart.

Another question - is a computer only CPU and Memory? How about the Keyboard, Monitor, and stuff? Once they wire the CPU and Memory, the designers will connect other devices to the CPU. They do this so that the CPU can take input from the user of the computer or display the result of an operation to the user. Sometimes, the CPU can take input from other devices (Example: 'Hard Disk'). It can also place the results of operations into other devices. All these devices are known as Input/Output (I/O) devices. So your keyboard, mouse, monitor, printer, scanner, speaker, microphone, flash disks, cd roms, dvd rams and hard disks are all input/output devices.

There are a whole lot of issues with Input/Output (I/O) devices that is supposed to be covered in Computer 301 (not even Computer 201). For now assume that the hard disk, the cd rom, the dvd ram, and flash disks are a special kind of input/output devices that are also considered to be part of Memory. (Tough nut to crack). This brings us to the details of Memory. As with every other thing in life, the best components in a computer system are the most expensive. For example, a type of memory known as 'Registers' are very fast (as fast as the CPU) but they are very expensive to produce. So we can't store too many instructions and data in them. Next in line is a type of memory known as the 'Cache' which is still expensive to produce. The best overall balance is achieved on a certain memory type known as the 'Main Memory'. The Main Memory can store a relatively larger number of instructions and data (although not permanently) and it is not too expensive to produce. It is also fast enough. The Hard Disk is known as a 'Secondary Memory' (as opposed to Main Memory) because it is far too slow but can store huge amounts of data permanently. So this is what the computer engineers usually do: They design the computer in such a way that the instructions and data that the CPU is currently working on are placed in the Registers; the instructions and data that the CPU uses most often are placed in the Cache; all (almost) the instructions and data for the programs we are currently working with are placed in the Main Memory; and lastly everything is placed in the Secondary Memory. You can think of the Main Memory as the workhorse of your memory system. It is with the Main Memory that the CPU has a special bond with. The Main Memory is also known as the 'Random Access Memory' (RAM).

Now things are getting tougher. So what exactly happens when you switch on your computer? After checking that everything is OK, the CPU loads a special program known as the 'Operating System' from Secondary Memory on to the Main Memory. Examples of Operating Systems are Windows XP, Windows Vista, and Linux. The CPU then starts executing this program. The Operating Sytem is designed to let you, the user, use the computer efficiently. So once the Operating System has loaded, let us say Windows XP, you can launch any other program by clicking or double clicking on a link. This again loads the new program into the Main Memory and the CPU then executes it.

Other Terms
Process: The bunch of instructions executing in sequence in the CPU constitute of a process. Usually, a single program will become a single process when executed.
Thread: Same as processes except that a thread is easier to handle for the CPU. A lot of technical details here.
Multiuser Computer System: A computer system that appears to handle more than one user at the same time. Remarkable!
Multitasking Computer System: A computer that appears to handle more than one process (or thread) at the same time. This act is also known as Parallel Execution.

Programmer: Someone who develops (writes) programs. Also known as coder or software developer.
Computer Geek: Someone who really enjoys talking and thinking about computers. (that's me)
Hacker: Someone who has a very deep knowledge about computers and loves making them do impossible things.

The Point ...?
This article is a compressed version of what students and researchers study about computers. I will be making use of ALL the terms and concepts I defined here in the future parts of the guide. These are the terms shown in bold italics. If there is anything unclear here, please feel free to ask.