What does a virus definition file contain?

Virus definition files contain information that allows an antivirus program to recognize files which contain a virus. Many simpler viruses can be detected by so-called "scan strings", which are essentially pieces of code found almost exclusively in a particular virus. Today, heuristic technology, which attempts to detect viruses without definitions, is also important. It operates by analyzing the structure of programs to detect if they may be harmful to a computer.