Memory

As you know, we use our brain to remember various things for short or long periods depending on the needs and necessities. Similarly, the computer uses its memory to remember the data and instruction. In other words, the memory of the computer stores data and instructions.
Similar to our house address, computer's memory (main storage) has a unique address.So, memory can be addressed with its unique address. The memories present in the computer are described below :
RAM (Random Access Memory) : Any location in the memory can be accessed at random by simply using the desired address, such a memory is referred as Random Access Memory (RAM). RAM is also known as read/write memory since data can be stored (or written) in or retrieved (read) from them. The disadvantages of such memory is that everything is wiped out (i.e. erased) once the power is shut off. 

ROM (Read Only Memory) 
Such type of memory in which information can only be retrieved or read from is known as Read Only Memory (ROM). Information is permanently stored in ROM, when it is manufactured. 

Types of ROM :
1) P.R.O.M. (Programmable Read Only Memory) :
P.R.O.M. allows user to write or program the memory, these P.R.O.M.s are very useful in micro computer applications.

2) E.P.R.O.M. (Erasable Programmable Read Only Memory) :
Unlike P.R.O.M.'s, the E.P.R.O.M. can be programmed. The content of E.P.R.O.M. can be erased by exposing it to ultraviolet light. The light passes through a quartz window on the I.C. and exposes the silicon chip which contains memory cells that are erased.
An important characteristic for both RAM and ROM is that the Access time (the time that computer takes to store or retrieve a data  item (word) in the specified (Address) location in/from the memory) is very little (50 Nano sec. to 1 Micro sec.).