What is Virus?
A chunk of computer programming code that makes copies of itself without any conscious human intervention. Some viruses do more than simply replicate themselves, they might display messages, install other software or files, delete software of files, etc.
A virus requires the presence of some other program to replicate itself. Typically viruses spread by attaching themselves to programs and in some cases files, for example the file formats for Microsoft word processor and spreadsheet programs allow the inclusion of programs called "macros" which can in some cases be a breeding ground for viruses.