A Radio Access Network is the part of a mobile network that connects user equipment, such as phones and modems, to the mobile core network over radio.
In 4G and 5G systems, the RAN includes radio units, baseband processing, scheduling, mobility handling, and interfaces toward the mobile core.
Open RAN disaggregates the RAN into components such as O-RU, O-DU, and O-CU and connects them through open interfaces.