Any storage unit of a computer system is characterized and evaluated based on following properties .
Storage capacity - It is the amount of data, which the storage unit can store, Larger storage capacity is desirable. As compared to secondary storage units, primary storage units have less storage capacity.
Access time - It is the time required to locate and retrieve stored data from the storage unit in response to a program instruction. Faster access time is preferable. As compared to secondary storage units, primary storage units have faster access time.
Cost per bit of storage. It refers to the cost of storage unit for a given storage capacity. Lower cost is desirable. As compared to secondary storage units, primary units have higher cost per bit of storage.
Volatile - If a storage unit can retain data stored in it even in case of power off or interruption of power, it is called non- volatile storage. On the other hand, if a storage unit loses data stored in it in case of power off or interruption of power, it is called volatile storage. Obviously, non-volatile storage is desirable. In almost all computer systems, primary storage units are volatile and secondary storage units are non-volatile.
Random access - If the time taken to access a piece of data from a storage unit is independent of the location of the data in the storage unit, it is called random access storage or random access memory (RAM). Each location of a RAM is as easy to access as any other location and takes the same amount of time. In almost all computer systems primary storage units have random access property and secondary storage unit have either pseudo-random access (access time is nearly same for all locations but not exactly same) or sequential access(access time depends on the location of the data) property.