The music varies enormously, but it all has the same basic rhythm in common. Although there are various rhythms within rhythms salsa is danced to a strong pattern of 8 beats comprising 2 bars. The first bar we label 1,2,3,4 - logically enough, and the second 5,6,7,8. The basic step is made on 1,2,3 and 5,6,7. The speed various about +-20% from 200bpm depending on the piece. There is a gargantuan repertoire of renditions from very percussion based music to very melody based music, and the instruments vary enormously, as do the singing styles. The vast majority of salsa music is not English, but the stupendous spectrum of available salsa music means that there is always something to please almost anyone. There must be hundreds of salsa bands around the globe - the music seems to cut across all cultures. The dancing definitely inspires your own interpretation of each piece and is why so many people get a kick out of expressing themselves on the dance floor. When a piece of music "does it" for you, then it just carries the dance. I quite often catch myself sing along to lyrics to which I have no conception of their foreign meaning!
For a more technical definition salsa music is defined as containing certain percussion instruments. The purist defines salsa as containing the clave, though most salsa dances would say the Conga, played on the Tumbao, (pronounced Tumbow - the "bow" as in a bending forward salutation) gives them the rich timing that makes them dance well. The clave is an instrument and a percussive pattern - basically two hard short thick sticks banged together on 2, 3, 5, 6.5 and 8. It contains all the timing information required to dance perfect salsa, though on it's own is a little tedious! The Tumbao is a bit like a tall bongo, and the Conga rhythm is 16 hand-hit "beats" over 2 bars: 8 hits are marked for each bar (i.e. 16 hits in a full basic step) - though it still only marks 4 beats of music in each bar, it's just that it supplies extra rich timing information in the half beats. As a side note, this is why when one gets better at salsa dancing, the preference is "proper" salsa music as opposed to R&B for example (R&B doesn't have the same rich timing content). The Conga is played with sequence of 4 different sounds made by different hand positions when hitting the hide. There are a number of Conga rhythms, but the vast majority naturally mark 8 (or 4) and 2 (or 6). A 2-drum Conga more easily identifies the difference between bar 1 and bar 2 (the 2 drums being tuned differently), so this sort of pattern is easy to follow and stay in time with. The 8 (or 4) [and commonly the 8(4) and 8.5(4.5)] get an "open" hit, which sounds like a "bong", an the 2 and 6 get a "pop" which is a loud popping sound, but dead with no resonance. When the pop is dominant in the music, salsa feels nice danced on the 2 [same as 1, but with the break on 2 - the sequence of weight transfers is identical, it's just a different spatial pattern]. There are other instruments like the cow-bell, which marks 8, 8.5, 1, 2, 2.5, 3, 4, 4.5, 5... etc, and some variations, and a washboard sound (which I can't remember the correct name for). Listening to proper salsa music, and identifying 1 by using the melody is usually enough to the retrofit that on to the percussion and use that for your timing in the dance. Of course the lovely melodies, breaks, transitions, bridges and phrases in the music is what drives your style and attitude whilst dancing, and this is expression of your inner interpretation when dancing salsa.