Drop rate

From WoWWiki

Drop rate is defined as x of x-kills that yields a specific item.

An example:

You kill 200 ogres and get 100 Netherweave Cloth, then the droprate is equal to: 100 / 200 = 0.5 which is equal to a 50% droprate.

Another example:

An item has a 1% droprate from a specific mob, which translates that it will drop in every 100 kill. It doesn't mean though that it's guaranteed to drop every 100th time, but it's an estimate; it could drop the 10th kill or the 120th.

A calculator to determine how often you will have to kill a mob can be found here: http://ah-web.at/mobs.php