JASS, GUI, JESP, UMSWE GUI, and vJASS
It is unclear what JASS stands for exactly as it was a term named by Blizzard. Most people believe it means "Just Another Scripting Syntax" or along those lines. WC3 actually uses JASS2 as JASS1 was used by StarCraft I.
GUI stands for Graphical User Interface. It was named such because you used a Graphical User Interface to make the triggers as opposed to typing script.
UMSWE I am not sure of, the people at WarCraft3Campaigns made that.
vJASS was an extension to JASS made by people at WarCraft3Campaigns (specificly Vexorian) during the production of JassHelper.