Glass cannon refers to a character who deals out huge damage but is unable to take much in return. This term most often applies to mages, who tend to have very high killing power in the Diablo series, but very low hit points/blocking/defense, and other survival boosts. Diablo 2 Bowazons were also fairly renowned glass cannons, with their huge ranged attack damage but low defense, hit points, and no blocking.
Melee characters can be glass cannons as well, though they should probably get a different term, just since "cannon" is associated with ranged attacks. The Diablo Hellfire Monk was something of a glass cannon; he could hit multiple enemies at once, very quickly, for very high damage, killing much more quickly than the Diablo Warrior. But the Monk had much lower hit points, and was only effective when using a staff, thus depriving him of the defensive bonuses provided by a shield.
See the full list of Abbreviations, Acronyms, and Game Terms in the Diablo 3 wiki.