A hard drive is a device inside a computer that lets you save documents, applications, images and more, and also lets you open them again. Inside it has a disk and an arm, that reads and writes information onto the disk. If your hard drive is damaged, you will loose everything on your computer. SSD stands for solid state drive. An SSD is like a hard drive but it is smaller and lighter, and contains no moving components inside, unlike a hard drive. This means that it is less likely to break if dropped.
An optical drive uses a laser to read and write information on a disc. You can play DVD's CD's and Bluerays. While optical drives spins disks very quickly, they are still a lot slower than hard drives which store data magnetically. Optical drives have been used since the 1900's and are used in all computers.
RAM stands for random access memory. Unlike a hard drive, your applications are only temporarily stored onto a RAM, to allow you to access them quickly. The more gigabytes the RAM has, the quicker you will be able to open documents, paste picures or open applicatios.
Everything in the computer is connected to a Motherboard. The Motherboard contains a fan, to stop the processorheating up to much. This is why you often hear a whirl when you turn on the computer. The RAM slots, USB connectivity and the processor are in the motherboard.
CPU stands for cental processing unit, and it is the brains of the computer, where all the calculations take place.