What is GD?
GD is a computer programming library used by many programmers to perform image manipulation/creation tasks. It is very commonly used in php for tasks such as resizing images or creating a CAPTCHA image ( a measure used to prove that you are a person and not a robot ).
The letters in GD had at one point in time meant "gif draw", after some patent issues with the GIF image format and the fact that the GD library does much more than just process gif formatted images, the GD does not really officially stand for anything. On GD's website, libgd.org, they say for all intensive purposes that the GD can just stand for "graphics draw".