Rotation is a common term used by many dps classes when it comes to raiding. The basic idea is that you have a sequence of abilites to use, that you then repeat, or rotate; hence the term.
Rotations are designed to increase your max dps, and for some classes there's quite a bit of debate as to which rotation is best. The ability to properly execute a rotation is a basic skill a raider playing a dps class must master.
See Shot Rotation.
They are usually described in a short form, like 1s/5r, meaning you must first get 1 combo point for Slice & Dice, then 5 for Rupture, and repeat.
Your rotation is subjective to your hit and OH weapon speed. If you have a high amount of hit and a very fast OH you can get away with 3s/5r. If your OH is slower and hit is lower then you will want to keep a 4s/5r or 5s/5r to guarantee maximum uptime of SnD. The gear level of the rogue is the main indication to which rotation he should be using.
The short form for the finishers are as follow:
Popular rotation include:
Shadow Priests do not have a true rotation like either of the classes described above, but rather a spell "priority". This is detailed out in the shadow priest guide. Essentially, the priority is the following: