Many free operating systems are available. They are called Linux operating systems. There are over 300 different Linux "distributions". Each one has a different purpose. Ubuntu-Linux is the most-used Linux because it is the easiest to use.
A modern PC has a minimum set of parts to be useful. The "Base unit" or "Tower" is the main part of the computer. A mouse and keyboard are used for input. A monitor is needed to view output. In a laptop computer these parts are all together.
Inside the base unit or tower there are many electronic parts. The main parts are the motherboard, CPU, the hard disk and memory.
The CPU (Central Processing unit) follows the instructions in the operating system and application programs. The memory, or RAM (random access memory), is for moving information (or data) quickly to and from the processor. The hard drive holds programs and data while the computer is powered off. Floppy drives, CD-ROM drives are used for storing information on removable disks.
Computers often have a modem to send and receive data over a phone line, or more recently, over cable television lines. Computers can also be connected to a computer network.

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