Only the CPU has access to this memory. These instructions are burnt into the chip with the help of special equipment before the chip is installed inside the computer. Users do not have access to the ROM, so they can't change these instructions. RAM is a collection of cells and we can access any cell at any given time, hence the name random access memory. RAM is often referred to as main memory or primary storage.
RAM usually occupies a number of chips connected together.
It is used to store programs and data being used by these programs while the computer is on. That is why RAM is often called volatile while ROM is a non-volatile type of memory. Every cell in the memory is assigned a unique number called a memory address. It is hard to remember memory addresses as numbers so many programming languages allow to refer to cells with alpha-numeric names. Every time we refer to the cell, the copy of its content is used by the instruction.