RAM

RAM is also known as Computer Memory. This is short term temporary memory which will be erased as soon as the computer is switched off. It only deals with the things that are happening at the moment.

Hard Drive

Hard Drive is the permanent storage space where all your documents, photos, music, etc. are stored. It is long term memory and stays in place when you switch off your computer.

Processor

This is the brain or the hard working part of the computer. It is doing all the tasks that you have asked the computer to do. It sorts things out, moves information, does the calculations, presents information to the screen or printer and so on.

Simple Explanation

Lets imagine that the number of hobs on the cooker in your kitchen is like the RAM and you are the Processor, cooking 10 different dishes for 10 people. Now, you can start cooking different dishes simultaneously depending on the number of hobs on the cooker. So, the more hobs you have, the faster the cooking will be done. This is exactly how RAM works. More RAM means more applications (or tasks) can be processed simultaneously and more quickly by the processor.

When you have finished cooking, you will store the food in a pot until it is needed. So, now the pot represents the Hard Drive - it's just storage. That's all.

Why can't I fill my computer with RAM?

This might make you think, why can't I add as much RAM as I can afford? But, here is the catch. Imagine your cooker as a computer and it can take only so many hobs. You can't just add as many as you like. Similarly, each computer can take only so much of RAM.

I hope you understood the basic concept of RAM, Hard Drive and Processor.