What are the differences between a GD and a GD2 image?

Reading the documentation for imagegd2(), and imagegd(), I noticed the functions are described, respectively as:

• Output GD2 image to browser or file
• Output GD image to browser or file

What are a GD2, and a GD image? What are the difference between those image types?

gd is an old c library, gd2 is the updated one. php uses gd2.