In the PRAM model, there are an unbounded number of processors and a single memory. Although the processors operate in lock-step by executing one instruction per time step, each processor maintains its own state and can therefore execute the program in an arbitrary way according to the control flow.
In a CREW PRAM, two processors can read from the same memory location in the same time step, but only one processor can write to any memory location in one step. In an EREW PRAM, reads can also not occur concurrently.