The GTFS specification defines the following files along with their associated content:

Filename Required Defines
agency.txt Required One or more transit agencies that provide the data in this feed.
stops.txt Required Individual locations where vehicles pick up or drop off passengers.
routes.txt Required Transit routes. A route is a group of trips that are displayed to riders as a single service.
trips.txt Required Trips for each route. A trip is a sequence of two or more stops that occurs at specific time.
stop_times.txt Required Times that a vehicle arrives at and departs from individual stops for each trip.
calendar.txt Required Dates for service IDs using a weekly schedule. Specify when service starts and ends, as well as days of the week where service is available.
calendar_dates.txt Optional Exceptions for the service IDs defined in the calendar.txt file. If calendar_dates.txt includes ALL dates of service, this file may be specified instead of calendar.txt.
fare_attributes.txt Optional Fare information for a transit organization's routes.
fare_rules.txt Optional Rules for applying fare information for a transit organization's routes.
shapes.txt Optional Rules for drawing lines on a map to represent a transit organization's routes.
frequencies.txt Optional Headway (time between trips) for routes with variable frequency of service.
transfers.txt Optional Rules for making connections at transfer points between routes.
feed_info.txt Optional Additional information about the feed itself, including publisher, version, and expiration information.

The following image shows the relationships among the various files: