What is the difference between bug and glitch

They're NOT the same thing.

Official defenition:

Glitch: In video games, a glitch is a programming error which results in behavior not intended by the programmers. Glitches may include incorrectly displayed graphics, collision detection errors, game-freezing or crashes, sound issues, and others. Some glitches are potentially dangerous to the game save data.

Bug: A software bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from behaving as intended (e.g., producing an incorrect or unexpected result). Most bugs arise from mistakes and errors made by people in either a program's source code or its design, and a few are caused by compilers producing incorrect code

Allmost the same...

It is generally used like this:

In MMOs:

Glitch is a mistake in programming the game. Glitches are things, such as walking through walls.

Bug is something that a player can abuse to improve his in-game resources, or make a game easier and simpler to him.

In online(flash) games:

Glitch: Going "Out of bounds", going throught walls etc. and graphical errors.

Bug: Everything else.