As defined by Webster's Ninth New Collegiate Dictionary (Springfield: Merriam-Webster, 1984):

philately \fə-'lat-əl-ē\ n [French philatélie, from phil- + Greek ateleia tax exemption, from atelēs free from tax, from a- + telos tax; akin to Greek telein to pay, tlēnai to bear; from the fact that a stamped letter frees the recipient from paying the mailing charges] (ca. 1865) : the collection and study of postage and imprinted stamps : stamp collecting — philatelic \'fil-ə-'tel-ik\ adj —philatelically \-i-k(ə)lē\ adv philatelist \fə-'lat-əl-əst\ n (ca. 1865) : a specialist in philately : one who collects or studies stamps

A few good links to philatelic organizations follow.  Many more possibilities can be found, however.  Countries' postal services provide an excellent source of information.