There are over 400 Bus Lines covering the entire Paris metropolitan area stopping at more than 12.000 stops. They start to run around 7am in the morning. The majority of buses usually continue until 8:30pm and some others until midnight time. For some specific destinations, like the international bus station Gallieni, there are extra lines (called Noctilien) serving all night long. During the day hours, the frequency of the buses is between 5-8 minutes.
The tickets are the same with the Paris Metro tickets that you could buy from machines and cabins at all Metro stations. A single ticket would cost 1.90€. But if you buy 10 at once (Carnet in French) 16.90€. Additionally, passengers can buy tickets from the drivers as well for 2€ for each. Unlike the metro, multiple bus rides cannot be combined with the same ticket.
The public buses in Paris are all numbered with double digit codes like 24 or 87. And you can always read this number and the final destination of the line at the front of the vehicles.
How to get on a bus in Paris? First, they take only from the stations and if someone waiting wave a hand when they are approaching. Use the front door and validate your ticket through the machines inside. If you forget, you might be fined if you get caught.