Beginner

Entropy Encoding:
To reduce the amount of data lossless processes can be used. These methods make use exclusively of the redundancy of the data. Processes with this principle are called entropy encoding. There are several of this kind. Entropy encoding is used regardless of the media’s specific characteristics. The data stream to be compressed is considered to be a simple digital sequence, and the semantic of the data is ignored. Entropy encoding is an example of lossless encoding as the decompression process regenerates the data completely. The raw data and the decompressed data are identical, no information is lost.
 

Runlength Encoding:
Run length coding is an example of entropy encoding.
If a byte occurs at least four consecutive times the number of occurrences is counted. The compressed data contains this byte followed by a special flag, called M-byte, and the number of its occurrences. The exclamation mark “!” can be defined as this M-byte.
A single occurrence of this exclamation mark is interpreted as M-byte during the decompression; two consecutive exclamation marks are  interpreted as an exclamation mark occurring within the data.

Example: Runlength Encoding:
An example of a run length encoding:
Uncompressed data: ABCCCCCCCCDEFGGG
Compressed data: AB C!8DEFGGG