Western classical music finds its roots in early chant. At first, the church would only allow unison vocal music... Eventually allowing fifths and fourths. As we approach the Rennaisance period they began to embrace the "Sweet Sound" which was the inclusion of the third in the chord, some say was first written by Dufay. As more people outside the church began writing their compositions down, we see more complex chords and rhythms, we see polyphony - multiple interweaving lines - and the organ is allowed in church. By the Baroque period, instruments are loud enough and standardized enough we see small court ensembles. Nobles would pay to have a composer on staff, and composers like Handel are hugely popular. And we finally get our first conductor - Jean Baptiste Lully. He would keep time by striking a large stick on the floor. He accidentally stabbed his foot with the stick, and died of gangrene shortly after. But now we have the first orchastras as we know them today. Instruments of the time still had a long way to go to the instruments we know, chord progressions gradually become more complex, and the chords themselves become more complex. I hope this helps to answer the very broad question you asked!