What are the differences between Mozart's and Bach's music?

They are vastly different. Most of Bach's music was composed in an era of Aristocracy and Church. Mozart, on the other hand lived during the age of "Enlightenment" where people began asking questions about everything like science, math, and social issues. Mozart (and others during his time) were composing music in a new form. Bach composed "Fugues" where you have 3 or more voices moving horizontally and each has equal importance. Mozart (and his contemporaries) were experimenting with new things. The Fugue was considered old hat in his time although Mozart had great respect for Bach. If you listen to their music one after the other you would see that Bach is "Stuffed Shirt" but Mozart is free and more inventive with his harmonies. Bach was strict and adhered to voice leading rules of his time.