(I sense homeork question, oh well) Well, There's a huge difference between early baroque and late classical but between late baroque and early classical there isn't a huge distinction. Usally it's thought that the classical era begins with the developement of sonata form but there really is no definitive starting point. Until Haydn it's fuzzy what exactly is classical era music. Baroque is extremely structured and governed by lots of rules which they force down your throat in music school and then tell you they are almost all out dated and pointless. Classical was a little more free but it was governed by many musical forms such as sonata, rondo, tetiary and many others (eventually people threw these out but they had a revival in the twenith century). The Piano became the main keyboard in the classical while in the Baroque it was the organ or the Harpsichord. That's about it. Oh wait? Why there needed to be a change. I'll give you an example. Domenico Scarlatti wrote 555 (or something like that) Piano sonatas each almost the same length and in almost the same form (which many of are great pieces of music). but doing something like this or Bach's 300 and something chorales or Vivaldi's 40 somewhat operas becomes to standard to write almost the same music as these great composers so classical composers started to change.