Here’s an analogy I’ve used often to describe to people what happens when a computer is low on memory. I cannot fully claim it for myself – I heard the concept somewhere, and perfected the story to make it my own.
If your computer where a kitchen, the chef is the processor. No work happens until the chef can get it done, at whatever speed the chef can operate.
The Random Access Memory (RAM, or often called the memory) is the counter space. The chef will lay out all the ingredients here and work with them, and put them away when finished. When the kitchen in closed (computer is shut down for the night) the counter top is cleared off. If the kitchen is properly shut down, the chef will put away all the ingredients and wash up the dishes before leaving. If something were left on the counter, the cleaning crew will just throw it away rather than put it back (that is like what happens when you simply power-off a computer without properly shutting it down first)
The hard drive is the longer term storage area – like the cupboards, pantry, and refrigerator. You keep the ingredients, pots, pans, etc – all of the things you use, stored in there overnight or longer.
When you don’t have enough counter-top space (think RAM), you then need to “swap” items (think information, files, programs, documents, etc) back and forth. The chef can grab a couple of ingredients to make a meal, put them on the counter, work with them, and put them back and grab two more, running back and forth until the job is done. This is less efficient, but possible. If the chef has all the ingredients, bowl, spoons, mixer, and whatever else he needs to prepare the dish, he can stay in one place and get it all done at one time, getting it all out and putting it all back in one trip.
So when you are low on memory, you might be low on one of two things. The more common thing that I see is that you are low on counter space (or working room for Windows and the programs you want to run to operate together at the same time). Your processor is having to run back and forth to the hard drive to put one thing away and retrieve another, because there is not room to spread it all out and work with it at once. If you are low on Hard Drive space (your storage places in the kitchen) then you run into entirely different problems, which are not nearly as common to encounter (at least in my experience)
So my analogy is not entirely perfect. Recently, Dual-Core and Quad-Core processors have come about and that means that it is like having multiple chefs running around. Windows VISTA has introduced the option to plug in a USB drive and treat it like additional RAM (I guess setting up a card-table in the kitchen and using it temporarily). But there it is so far.
Any ways to improve the analogy?. Email me, or leave them in the comments. I’d love to see this story perfected some more.