Philately is the study of stamps and postal history, as well as other postage related items. It also refers to the collection, appreciation and research activities on stamps and other philatelic products. Philately involves more then just stamp collecting, which does not necessarily involve the study of stamps (but rather accumulation). 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. Here is a great book on the subject.
Also, if you are serious about philately and stamp collecting as a hobby, you will need the Scott 2018 Standard Postage Stamp Catalogue. This is Volume 1, for United States. The catalog is essential for pricing stamps. I know its a bit expensive, but you do not have to buy the same year for all the volumes. I have a Volume 5, for Russia and area, and it’s from 2000. And my Volume 1 is from 2015. I know I do not have the last 17 years of the Russian stamps in the catalog, but its only 17 years. In philatelic times, its nothing but a finger snap.
Almost all of the philatelists are stamp collectors.
Not all the stamp collectors are philatelists.
See also: Tools of the Trade