The Paris Transit System is a comprehensive network of public transportation that includes the METRO, which is a rapid transit system with five letter lines. The METRO serves the city of Paris and its suburbs, providing an efficient and affordable way to get around the city. It is one of the most heavily used public transportation systems in Europe, with over 1.5 billion passengers using it each year. The METRO is also connected to other forms of public transportation, such as buses, trams, and RER trains, making it easy to travel between different parts of the city.