The memory of a computer can hold program instructions, data values, and the intermediate results of calculations. All the information in memory is encoded in fixed size cells called bytes. A byte can hold a small amount of information, such as a single character or a numeric value between 0 and 255. The CPU will perform its operations on groups of one, two, four, or eight bytes, depending on the interpretation being placed on the data, and the operations required.
There are two main categories of memory, characterised by the time it takes to access the information stored there, the number of bytes which are accessed by a single operation, and the total number of bytes which can be stored. Main Memory is the working memory of the CPU, with fast access and limited numbers of bytes being transferred. External memory is for the long term storage of information. Data from external memory will be transferred to the main memory before the CPU can operate on it. Access to the external memory is much slower, and usually involves groups of several hundred bytes.