What is GTFS?
The General Transit Feed Specification (GTFS) is an open data format for transit schedules and service data.
GTFS was developed by Google and Portland TriMet, and was originally known as the Google Transit Feed Specification. GTFS data is produced by hundreds of transit agencies (including Sun Tran) to deliver content for inclusion in maps and transit navigation services, including Google Maps.
A GTFS feed is typically presented as a series of text files with comma-separated values that conform to the specification. Each file is essentially a table in the relational database that is the whole package.
Learn more about GTFS on the Google developers site.