There are several means to compress data. Here, we are only going to deal with
the losslessy schemes. These schemes are also called non-destructive because
you always recover the initial data you had, and this, as soon as you need them.
With losslessy schemes, you won't never lose any informations (except perhaps
when you store or transmit your data but this is another problem...).

In this introduction, we are going to see:
- The RLE scheme (with different possible algorithms)
- The Huffman schemes (dynamical scheme)
- And the LZW scheme

For the novice, a compresser is a program able to read several data (e.g. bytes)
in input and to write several data in output. The data you obtain from the
output (also called compressed data) will - of course - take less space than
the the input data. This is true in most of cases, if the compresser works
and if the type of the data is correct to be compressed with the given scheme.
The codec (coder-decoder) enables you to save space on your hard disk and/or
to save the communication costs because you always store/transmit the compressed
data. You'll use the decompresser as soon as you need to recover your initial
useful data. Note that the compressed data are useless if you have not
the decoder...

You are doubtless asking "How can I reduce the data size without losing some
informations?". It's easy to answer to this question. I'll only take an example.
I'm sure you have heard about the morse. This system established in the 19th
century use a scheme very close to the huffman one. In the morse you encode
the letters to transmit with two kinds of signs. If you encode these two sign
possibilities in one bit, the symbol 'e' is transmitted in a single bit and
the symbols 'y' and 'z' need four bits. Look at the symbols in the text you are
reading, you'll fast understand the compression ratio...