A virus in a computer is a piece of code that is capable of copying itself and damaging software.
A can be a damaging thing that can corrupt the system, or destroy data.
Basically a virus is a piece of code that is capable of copying itself, it can also be a program, which is loaded onto a computer typically with the purpose of causing harm to the system.
All computer viruses are man-made and can be relatively simple in their construct, but even a simple virus is capable of copying itself and using up system memory which will bring the computer to a halt.
How are viruses spread?
More dangerous viruses are capable of moving themselves across networks and bypassing security systems.
Computer viruses are extremely common and can be spread by things like attachments in emails from un-trusted, or sometimes even trusted sources that are unaware that their system has been infected by a virus.
Numerous antivirus programs exist to help users protect their systems from infection, these can be accessed as both free software and paid for software.