How does a Computer work?
Computers are ditial systems. So first lets explain what it means for a system
or device to be "ditial". Being ditigal means that a system stores and operates
on information in a very specific way. In a digital system, every piece of 
information is stored in a bit (or multiple collections of bits). A bit is a 
variable that can have only one of two values: it can either be a 1, or be a 0.
An easy way to think of a bit is to think of a switch - it is either on (bit =
1) or off (bit = 0) there is no between.

Any piece of information can be represented as a collection of bits. The way 
this is done is by making each bit the answer to a yes or no question. For 
example, lets say I wanted to know whether a person has long hair or not. I 
could represent this piece of information by deciding that if the person has 
long hair, I will set a certain bit to be a 1. If the person doesn't have long
hair, I will set that same bit to be a 0. As long as you followed the same 
convention, you could look at the same bit that I just set and know whether 
the person has long hair or not, even if you had never seen them before.

The fact that a computer is digital means that it (or any other digital system)
can really only do three things:
1. Set a bit to be 0
2. Set a bit to be 1
3. Look at the value of a bit and use it to decide what value to give some 
   other bit. 

So how do computers do all of these complex things that they do? Well, computers
look at and manipulate millions and millions of bits. Each one of those bits is
assigned a certain meaning, in a certain context. When you tell a computer to do
something, all that is really going on is you are setting certain bits in a 
certain way, and the computer interprets those bits to figure out what you want
it to do. It's important to understand that the bits in and of themselves have 
no real meaning - it's only by assigning meaning to them that we can interpret 
the information they contain.

Because computers operate on such enormous numbers of bits, we group bits 
together and gives these groups their own names. A "byte" is simply a group of 
8 bits. A kilobyte is about one thousand bytes, a megabyte is about one million
bytes, and a gigabyte is about 1 billion bytes.

The Three Subsystems of a Computer
1. CPU
The Central Processing Unit, otherwise known as the CPU, looks at certain bits 
to determine what its being told to do, and once it has figured that out it 
performs that operation. The CPU is the brain of the computer - it does all the
interpretation and manipulation of bits.
2. Memory
Memory does exactly what its name implies - it remembers the values of bits. It
can either store bits sent to it by the CPU or return the values of those bits 
back to the CPU. Hard disks and RAM are really just two different kinds of 
memories; they both perform the same function, they just use different physical
mediums to do so.
3. Input/Output
The Input/Output (I/O for short) subsystem also does exactly what its name 
implies - it takes from the outside world and sends them to either the memory 
or the CPU, and it also takes bits and sends them back out. The keyboard and the
screen are part of the I/O subsystem.