Chicago Transit Authority
In short
The Chicago Transit Authority is the public agency responsible for operating the 'L' rapid transit trains and bus services in Chicago, Illinois.
In plain words
It is the main company that runs most of the public transport in the city. It manages both the underground and elevated trains and a large network of buses.
A simple example
A commuter can use a single Ventra card to pay for a trip on a CTA bus and then transfer directly onto a CTA 'L' train.
Why it matters
It provides essential movement for millions of people across the city every day. Without it, much of Chicago's economy and daily life would struggle to function.
Easy to mix up
Do not confuse it with Metra, which is a different agency providing commuter rail services to the wider suburbs.