What is Reverse Engineering?

From Wikipedia: Reverse engineering, also called back engineering, is the process by which a man-made object is deconstructed to reveal its designs, architecture, or to extract knowledge from the object; similar to scientific research, the only difference being that scientific research is about a natural phenomenon.

So, in other words, this is a process that we use to extract or modify valuable information from the client executable of the game to assist on either side. This wiki will cover both client-side (how to unpacked and patch a client) but also server-side (how to read expected packet results, for example).