Well, that is easy - the word precancel is self-explaining. A precancel is a stamp that is canceled when bought from the post office. The reason? Labor saving. Usually these stamps are used by mass mailers. The most common precancel used in the US are probably the two parallel lines with or without writing in-between but in the last twenty or so years we have gone to stamps with the service inscribed. These are not precanceled in the stricter sense of the word but do not exist without the inscription
Mooselaab, For some examples, just go to Google and enter "US Precanceled Stamps", then click on "Images" at the top of the page, and it will show you a boat load of what they look like. Once you're familiar with that, it won't be a problem for you to recognize them.