You will find stamps in a variety of forms. The same design that you can find on a single stamp could also be on a souvenir sheet, and/or part of a miniature sheet. You may also find a stamp, especially earlier ones, with a totally different peroration pattern. And just to make things even more complicated dealers often will only sell the complete set instead of breaking it up.
For example, if you collected stamps that featured spiders you would be on the lookout for stamps that had pictures of a spider or spiders. Lets say that you spotted a set of five stamps that feature spiders but you will need to purchase the whole set because no stamp dealer will split the set up and sell you the individual stamps that you need to complete your collection. You only want two stamps from the set because you already have the other three. What do you do with the unwanted stamps you were forced to purchase?
Discard them into your pile of material for trading purposes?
Mount them in a different album?
Try to resale them to another collector?
All of the above are possibilities. You have the final choice as to what you do with the unwanted stamps. You will no doubt locate sheetlets of stamps of different designs where at least one stamp will feature a spider and the other stamps of the set do not. In a case like this your best bet is to keep the sheet intact. Most collectors would rather purchase the set intact as opposed to buying the stamps individually.
In a simpler time, stamp collecting consisted of collecting stamps, each issued in sheets that contained multiple copies of the same design. The only varieties were from a different perforation measurement, different paper, or a change in the stamps color or shade.
Stamp collecting is a lot more complex today. Here are a few configurations you are likely to run into:
Single stamp--This is a traditional stamp. One stamp with something in its design that causes you to want to include it in your topical collection.
Setenant--Two or more stamps with different designs printed together on the same sheet.
Block--An even number of stamps--4,6,8, and so on--still attached together.
Margin block--A block of stamps from the edge of the sheet, with the unprinted paper on the edge, or margin of a sheet of stamps.
Plate number block--A block of stamps from the edge of a U.S. pane of stamps that includes the plate number designation.
Sheetlet--A small sheet containing more than one stamp