A computer virus is typically a short program designed to disperse copies of itself to other computers and disrupt those computers' normal operations. A computer virus usually attaches or inserts itself to or in an executable file or the boot sector (the area that contains the first instructions executed by a computer when it is started or restarted) of a floppy disk,cd-rom or other media

Although some viruses are merely disruptive, others can destroy  or corrupt data or can cause an operating system(like your Windows)or applications program to malfunction.
Computer viruses are spread via floppy disks, networks,cd-rom discs,usb pen drives, on-line services or other forms of media. Several thousand computer viruses are known, and on average three to five new strains are discovered every day. Virus programs can also infect advanced cellular telephones

And how can we PROTECT our COMPUTERS against them.?

Back up important files on floppy disks, cd-rom, or other storage devices on a regular basis, so that they can easily be replaced if a virus wipes out your hard drive or corupts your file.

Install the latest antivirus software on your computer, which can be purchased from a retail store or downloaded of the web. Check our link pages for some antivirus program links

Check your antivirus software vendor's website (and windows updates regularly if your using MS windows)for updates that will protect against viruses and worms.

Don't open email from strangers or attachments you weren't
expecting—especially attachments with .exe extensions