Among graphic designers, the word instills fear. Among the rest of the population - confusion. Some people refer to them as cuts, as it's cutting the main part of an image from the background, so it can be implemented on another background. Creating a good void isn't as simple as it seems.
People have different preferences.
Basically, it's tracing around an object, so only the subject you want is outlined, while the background is not showing. The Extract tool allows you to set the thickness of the brush, and all the preset zoom levels. You can stop at any-time to give your hand a good shaking, and the Eraser tool is handy. The other method is the lasso tool. From my experience, you can't stop or else it stops tracing, although I've only used this sparsely.