1. Usually refers to the casting classes, i.e. the ones that rely on magic instead of melee.
2. Generally the term applies to any character class that delivers more attack damage than, yet will nevertheless lose in a 1 vs 1 fight to, some standard garden-variety warrior.
A build of an Amazon in Diablo II, which has low life and defense but makes up in the fields of insane damage and speed with a bow.
You basically stack Dex instead of Vit, and instant damage.
Your goal is usually a 7 frame attack.
Fred: Woah, did you see Bill's new Glass Cannon? That mother fucker owned all the little hoes in that pub game!
John: He stole my gold :(
Fred: Shut up John, you little bitch.