Notre Dame's Choir
After the nave was mostly done, the builders worked on the crossing, then the apse. They finished the apse in 1182 AD, and the nave in 1196. Then they began working on the transept, but they soon ran out of money to pay the builders and to pay for the stone. Then the Bishop of Paris, Maurice de Sully, died in 1196 AD. The whole thing had been Sully's idea, so when he died, it took a while to get going again. In the end, the transept and the front towers were not finished until 1267 - a hundred years after they started work on the cathedral.
But by that time, the people of Chartres had already built their cathedral, which was higher than Notre Dame and had bigger windows that let in more light. Even though Notre Dame was just finished, it seemed a little old-fashioned. So the builders ripped out the small round windows all down the nave, and put in bigger windows, except for the walls right next to the crossing, where the walls needed to be stronger. There you can still see the original small windows (although, because they are blocked by other supports, these windows don't let in any light).
At the same time, the builders also added a lot of little chapels to the sides of the cathedral, along each of the side aisles, and also around the apse. This was because it had become the custom that each priest should say Mass separately every day, and all the little altars in the little chapels made room for all the priests to say Mass.
Even before the cathedral was finished, in 1185, Heraclius of Caesarea used it to convince people to go on the Third Crusade. Once the cathedral was finally finished, people began using it for all sorts of things - large public lectures, public ceremonies and meetings. When Louis IX came home from the Seventh Crusade with the Crown of Thorns, he kept it in Notre Dame while he was building the Sainte Chapelle (and actually they keep it in Notre Dame now).
Learn by doing: visit a big Catholic church and hear Mass
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