ODU (Out-Door Unit) refers to the set of satellite equipment which is placed outside of the building.
The dish receives the signal and focuses it on the feedhorn. The feedhorn delivers the signal to the LNB. The LNB converts the satellite frequencies to an Intermediate Frequency (IF) which is easier to transmit over the IFL (Inter-Facility Link) to the IDU. The BUC, if present, operates in the opposite direction and converts the IF signal from the IDU to the high frequencies required for transmission to the satellite.
In many ODU systems multiple logical devices will be integrated into one physical device. For example, an LNBF is a Low Noise Block (LNB) with an integrated feedhorn.