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Most computers possess four kinds of memory:
registers in the CPU,
caches both inside and adjacent to the CPU,
physical memory, in the form of RAM which the CPU can write and read to directly and quickly; and
disk storage, which is much larger, but also much slower
Most applications require access to more information like the code and the data than that can be stored in physical memory. This is especially true when the operating system allows multiple processes/applications to run seemingly in parallel. The obvious response to the problem of the maximum size of the physical memory being less than that required for all running programs is for the application to keep some of its information on the disk, and move it back and forth to physical memory as needed, but there are a number of ways to do this.
