are there bass, tenor, alto and soprano trombones?? what are their differences and, which one is the one people usually learn?

theres definately a bass alto and tenor trombone, ABRSM has grades for Tenor 1-8 and Bass 6-8 Alto is more a virtuoso kinda thing sort of like a flautist playing piccolo oboist playing english horn bassoonist playing contrabasoon etc

Historically informed performances of music from Mozart to Schumann often need an alto trombone. Sometimes it can be extremely useful. Schumann's "Rhenish" Symphony takes the first trombone up to Eb at the top of the treble clef, and although good tenor players can get there they tend to sound too heavy. Schumann gains a lot from replicas of the original instruments, because his scoring sounds very heavy on modern, wide-bore brass.

Soprano trombones have been made, and soprano sackbuts still are, but they were rarely used in Renaissance music (if indeed they existed then); the cornetto was more agile and had a sound that complemented the sackbuts very well. A trumpet with a single slide is thought by some experts to have existed in the 15th C. and one with a double slide in Purcell's time, but both proposals are disputed. No examples survive.