Anything from the name of potion to the way enemies respond when you attack them can be edited with a simple program. There are also hundreds of tools available for you to change things like EXP, Strength of enemies, names of enemies, names of weapons, enemy AI, spells - names of anything, and any text anywhere in the game. There are also a million other mods - just about everything in the game is now moddable, from avatars to the text. It's a shame, because they looked amazing in FF7, especially in battles. It's still possible to do this yourself but it was banned on the modding forums due to the illegality of it.
The best phase was when Crisis Core and Dissidia came out - all the character models were being ripped from Dissidia, CC, Dirge, Kingdom Hearts and put into the game. There are hundreds of different model types to choose from now - AC Cloud, etc. There is a mod that has all the lego characters replaced with either fan made or battle models.