The tactic itself involves utilizing quick reflexes to spot an enemy, center them on screen as best as possible, and then zoom into your scope only to then fire as soon as the crosshairs are visible.
Some view it as an exploit. This is due to the fact that the crosshairs essentially "lock on" briefly and allow one to achieve relatively easy one-shot kills. A missed shot usually means one need only quickly repeat. Since it's primarily dependent upon reflexes and simple visual cues, sufficient practice by any player eventually leads to the ability to quick scope repeatedly with a diminishing overall vulnerability while doing so.
The tactic is helpful for snipers while they're on the run, in close quarters combat, in place of a weaker secondary weapon. However, abuse of the tactic as a primary method of fighting has lead many to view it as the "cheap" preference of noobs. Many who rely on quick scoping have furthered this stereotype by abusing it repeatedly, even going as far as ridiculing those who snipe in more traditional fashions (often calling them "hard scopers" in a derogatory manner).
Quickscoping was founded in 1999 during a game mod known as Counter-Strike by guy guy named "CaLL Me God" and another guy named "Incorporated" They listed Quickscoping as a Technique used only for Close Quarters Combat. It was to be used only when sniping at Close range to medium range.
"Man, I was playing Modern Warfare 2 last night. I kept getting killed by this noob who did nothing but run around and quickscope. His hits were like 1 in 10, but he did it so frantically that he got enough kills to annoy us. His team still lost, though."