Which is fastest? Main memory or secondary memory?
Main memory.
Which is largest? Main memory or secondary memory?
Secondary memory.
There is a design trade-off between access speed and size. A computer would be tremendously expensive if all it had was a huge amount of main memory. It would be very slow if all it had was secondary memory. So modern desktop and laptop have both, and use each for what it does best.
Main memory is where programs and data are kept when the processor is actively using them. When programs and data become active, they are copied from secondary memory into main memory where the processor can interact with them. A copy remains in secondary memory.
Main memory is intimately connected to the processor, so moving instructions and data into and out of the processor is very fast.
Main memory is sometimes called RAM. RAM stands for Random Access Memory. "Random" means that the memory cells can be accessed in any order. However, properly speaking, "RAM" means the type of silicon chip used to implement main memory.
When people say that a computer has "512 megabytes of RAM" they are talking about how big its main memory is. One megabyte of memory is enough to hold approximately one million (106) characters. A mid-priced desktop has about 8 gigabytes or more of main memory (2018). A gigabyte is approximately one billion (109) characters. (There will be more about bytes and megabytes later on in these notes.)
Nothing permanent is kept in main memory. Sometimes data are placed in main memory for just a few seconds, only as long as they are needed by the processor.
Imagine that you are using a computer to write a book report on Tom Sawyer.