It is the part that avtually does the computing. In your machine is is likely a flat square about 1 1/2" on a side and about 1/8" thick.
Short course.
RAM = the memory actually used during processing. They will look like long cards with smaller rectangualr chips lined up on one or more likely two sides. There will be one or more near identical memory cards and they will be arranged in a bank.
Think of memory as the space on your desk where you work. Various things are taken out, worked on and then put back in storage.
Hard Drive = storage or filing cabinet. This is a rectangular device, about the size nwr posted. It is a shell and inside will be one or more disks that revolve at a moderate speed, usually 7500 RPM or less. This is were you keep all of the things that you are not using right now. When you turn your computer off, it cleans off your desk, the stuff you had in memory, in RAM. But all the stuff on the hard drive is retained, like what is in a filing cabinet.
Processor (AKA CPU) = You. It is what actually does the work. It gets things from the hard drive, moves them onto the desk (RAM), works on them and then puts the finished process away, either back on the hard drive or off to a printer or out onto the internet.
You can see some of these
here
Aslan is not a
Tame Lion