Postage stamps are pieces of paper, with glue or adhesive on the back. They are put on the corner of an envelope to pay the fee for having the postal service take the envelope to where it is being sent.
Philately is the study of stamps and postal history and other related items. It also refers to the collection, appreciation and research activities on stamps and other philatelic products.
Philately involves more than just stamp collecting, which does not necessarily involve the study of stamps. It is possible to be a philatelist without owning any stamps. For instance, the stamps being studied may be very rare or reside only in museums.
There are a lot of different postage stamps in the world. They have existed since the nineteenth century. Most countries have their own stamps. People collect them in books called stamp albums. Sometimes people collect stamps from only one country, or there is something else that links the stamps.