Your PC (Personal Computer) is a system, consisting of many components. Hardware components come in a number of different shapes, sizes and types in order to fulfill a desired task. A computer generally has all of the components below in order to work (bar the external hard drive).
The motherboard, also sometimes referred to as the main board, is a collection of devices (together on one large circuit board) that control the flow of data and operating electricity for all the primary components in a PC - for example, the CPU, memory, graphics adapter and sound card.
The CPU is basically the brains of the computer as it processes all the number crunching of the data. The faster that the processor can process calculation, the faster the computer will run.
Random Access Memory (RAM) provides space for your computer to read and write data to be accessed by the CPU (central processing unit). If you add more RAM to your computer, you reduce the number of times your CPU must read data from your hard disk. This usually allows your computer to work considerably faster, as RAM is many times faster than a hard disk.
Hard drives are the main device which stores all of the computers data. While most hard drives (HDD) have disc platters, super fast solid state drives with no moving parts are also available. Hard drives are available in 2.5" laptop size as well as 3.5" desktop sizes.
Optical drives have the ability to read CD/DVD/Bluray media or create discs by burning data to them, depending on the type of drive. With most software still coming on disc based media, an optical drive is important in order to install the data on the disc.
A graphics card is simply the component in a desktop computer that transfers a signal in the form of an image to the monitor or other display device. More expensive cards are generally able to process data much better, resulting in a greater performance boost especially while gaming.
A computer case is the outer shell in which all of the computer components go into.
The power supply unit (PSU) in a PC regulates and delivers the power to the components in the case. The more components (hard drives, CD/DVD drives, graphics card, cooling fans, etc) you have in your PC the greater the power required from the power supply.