It's hard to give a good generalization of what songs go with what dance because it can vary. And the same song can sometimes work with different dances. But a good example for foxtrot at least is Frank Sinatra music and stuff like it. Waltz is anything in three; if it's fast like music by Strauss (right?) then it's Viennese waltz. Rumba and cha cha are a little harder to pick out. Most cha cha music should have a distinct "cha-cha-cha" beat from the rhythm section in them, typically on the 4&1, but I'm sure there are exceptions. Rumbas are sort of like cha chas, but without that beat and slower. I bet if you web search you can find some lists of ballroom dance music that give you plenty of examples of what songs work for what dances.

Beats per minute is just what it sounds like -- how many beats of music there are in 1 minute of time. I think it's usually noted as note=# on sheet music, where note is the type of note that gets one beat and # is how many there are per minute, but it's been some time since I played from sheets with that on it. It tells you how fast/slow the song should be played. You can calculate it yourself for any song you want by playing it and timing how long it takes for a certain number of beats or counting the number of beats in say 10 or 15 seconds, then multiplying by 6 or 4 to get to a full minute. Ballroom dancing typically uses measures per minute as a tempo indicator though. Simple conversion (beats/minute) / (beats/measure) = measures/minute. I don't think you need to worry about this too much. If you're looking to buy music at a particular tempo for dancing, you should be able to go through ballroom dance supply stores to get music suitable for dancing without worrying about the tempo (it should all be fine). If you want to buy your own stuff from say popular CDs, try to web search out some ballroom music lists. If I'm not mistaken, there are some that also note which songs on particular CDs are good for dancing so you can use that as a guide for what to buy. I hope this helps.