I can think of five major groups right off:
PAL B/G/H - 625-line 50 Hz systems used in most European countries and many
countries in Africa, the Middle East and Asia.
PAL I - Again 625 lines at 50 Hz, but with a different sound carrier
frequency. Used in the United Kingdom, the Republic of Ireland, Hong Kong,
South Africa and a few other places I've forgotten.
PAL D/K - Yet another 625-line system with a different sound carrier. Used in
some former Warsaw Pact countries as well as mainland China; this may become
more popular as PAL/SECAM receivers become common in the CIS republics and
other former Soviet bloc countries, since those countries now use SECAM D/K.
PAL M - This one's 525 lines at 59.94 Hz; it's just like NTSC except for
PAL-type color encoding. It's used mainly in Brazil.
PAL N - A real oddity, with 625-line 50 Hz video but a LOWER sound carrier
and a lower color subcarrier than the B/G/H, I and D/K varieties. It's used
in Argentina and a few other places.