At the core of OpenTripPlanner is a library of Java code that finds efficient paths through multi-modal transportation networks built from OpenStreetMap and GTFS data. It can also receive GTFS-RT (realtime) data.
In addition to GTFS, OTP can also load data in the Nordic Profile of Netex, the EU-standard transit data interchange format. The upcoming EU-wide profile was heavily influenced by the Nordic Profile and uses the same schema, so eventual support for the full EU profile is a possibility.
GTFS and Netex data are converted into OTP's own internal model which is a superset of both. It is therefore possible to mix Netex and GTFS data, and potentially even data from other sources.