A kitchen full of RAM and HD
I often use a kitchen metaphor for RAM (random access memory) and HD (hard disk) that might be comprehensible to mere mortals, at least if they cook. RAM and HD space are both “memory” of sorts in your computer, but what is the function of each?
In your kitchen you have both counter space and cabinets. Your cabinets and refrigerator are where you store things you use to make meals. Your counters are where you do the actual prep work. You might pull a bowl and spoon from some cabinets, along with some ingredients to mix from the fridge, you do the mixing and cooking, then you put the bowl and ingredients away again. You might even store the leftovers of what you cooked back in the fridge.
If you imagine a kitchen with tons of counter space and very few cabinets, that would present a certain challenge. You could have four cooks in the kitchen, but you might not have the space to store four bowls. On the other hand, you might have a kitchen with tons of cabinet space, but very little counter space. Even though the chef may have every gadget imaginable available to aid in the cooking, he or she would still be bumping elbows into the walls and finding it hard to prepare a large meal.
Each element has a role to play: cabinets are great for long term storage, counters are great for getting work done. The key to a functional kitchen lies in the proper balance.
In your computer, the hard disk (HD, or these days maybe the “SSD” solid state disk) plays the role of the cabinets, the long term storage. The RAM (or “memory”) plays the role of the counters. RAM is where the work gets done, if you don’t have enough, the work slows down. HD is where the tools used to do that work and the product of that work get stored, if you run out of HD then you can’t properly save the work you’ve done or add new gadgets to make that work easier.
The metaphor could be extended, maybe the stove is the CPU, but let’s not get into that. For now, just think: my cabinets are the hard drive, my counters are the memory. You need enough of both to get the job done. Running out of either will either slow down the work or make a mess of the kitchen (as the pots come crashing onto the floor).