Some people like to collect stamps of maybe any one country, or several countries, but which are all of the same subject, or topic. These can be a lot of fun, and also an inexpensive way of getting to know about the stamps of many countries. Take birds , for example. There are thousands of different stamps showing birds. Space craft are not so common, but there are still many stamps showing those.
Sports is another topic shown on the stamps of all countries.
Stamps are usually printed by the Post Office in sheets containing many stamps. Sometimes there are 100 or more stamps on a sheet. In past days, all the stamps on a sheet were usually the same. In recent years, there can be several different stamps on the same sheet. Sheets often have a strip of paper around the edge, called selvage.