Hexing is the editing of .cat, .dog, .pet, .toy, and .clo files by players in order to modify them. The term comes from the hex editor software used by the first hexers before the release of LNZ Pro in 2006.
There are a number of programs specially made for hexing.
Most Toyz are made of animated filmstrips rather than the ballz and linez of other game components. Hexing these involves using a graphic editor like Photoshop to change each individual frame.
This wiki hosts a collection of hexing tutorials. They can be found here until I clean this up a bit.