Antivirus software is a program or set of programs that are designed to prevent, search for, detect, and remove software viruses, and other malicious software like worms, trojans, adware, and more.
Antivirus software can protect against known viruses. Some antivirus software scan files and compare a hash code for each file with its database of hash codes. If the code matches, it has likely found a virus. This way of doing things has some problems. It will only protect against viruses whose hash code (or "signature") is known. The companies who wrote the antivirus need to keep the virus signatures up to date and need to give this information to the PC that is to be protected.