Resource Block

Multiplex multiple users both in time and frequency, together with pilots and control signals.
The time-frequency plane is divided into chunks=minimum resource allocation unit.
The traffic multiplexing is performed by allocating to each user a certain number of chunks depending on its data rate/geometry.

Physical Resource Block
In OFDMA, users are allocated a specific number of subcarriers for a predetermined amount of time. These are referred to as physical resource blocks (PRBs) in the LTE specifications. PRBs thus have both a time and frequency dimension. Allocation of PRBs is handled by a scheduling function at the 3GPP base station (eNodeB).