A computer virus is a program that maliciously runs on your system without your knowledge. You get it by downloading fraudulent software, using infected thumb drives, or visiting unsafe websites. Signs that you may have a virus may include (but not limited to) programs running slower than normal, unusual popup messages, or new programs on your system that you did not install.
If you should suspect that your system has been infected by a virus either take it to a professional to have them remove it or install an antivirus program yourself and remove it. Should you decide to try to remove the virus yourself, make sure that you update the virus software before running it on your system so that it can download the latest virus definitions. In order to understand the virus definitions better, just think of them as a sketch artist that works for the police. Whenever the police needs to find a new suspect, they work with a sketch artist that will provide them with the adequate details in order to be able to accurately identify the "bad guys". That's exactly what the virus definition file is going to attempt to do.