Well, there are World Latin and Standard championships held every year (in varying locations), and the winners of those get to officially hold world titles for a year. However, I'd agree with the notion that Blackpool is considered by many to be the most prestigious event. The "problem" with Worlds is that each country can only send two (I think?) representatives to the event -- no such restriction at Blackpool, so all of the world's best couples are free to compete.
A few other competitions with international prestige that I can think of would be the German Open (500+ couples in the amateur events!), the UK Open in Bournemouth, and the Open to the World events at the United States DanceSport Championships. I've just pulled a few names off the top of my head -- there are others, of course. There are also many events that have great prestige for a particular area, as the Ohio Star Ball does for North America.
You ask about the difference between International and American Styles -- that's a much-discussed topic! You might check out the achives of the newsgroup rec.arts.dance (using deja.com) to see some of the discussion on that topic. To simplify, American and International are separate styles, with somewhat different technical approaches (I'm going to leave it at that regarding technique for this post). The dances used in competition are also different:
International Latin -- cha cha, samba, rumba, paso doble and jive
American Rhythm -- cha cha, rumba, swing, bolero and mambo
International Standard -- waltz, tango, foxtrot, viennese waltz and quickstep
American Smooth -- waltz, tango, foxtrot and viennese waltz
An easy way to differentiate Smooth from Standard is that you'll see open figures in Smooth (the couples can break apart and dance with no or modified connection). Couples remain in closed position in Standard.
Another difference between International and American styles is significant for those who compete: International Style are held around the world, and American Style events are essentially an American thing (some in Canada, and maybe a few elsewhere, but there's no American Style at Blackpool for instance). The names fit, don't they! A couple might choose International Style if they want to compete worldwide.
It's great that you're such a fan of dancing, Mark, but I definitely encourage you to experience it firsthand! If you're fascinated now, you'll probably fall completely in love with it once you get out on the dance floor
Vanessa