Most all antivirus programs use virus signature analysis to build a database that contains profile information belonging to known viruses, worms, trojans or other malicious code. As the list of known threats grows, new virus definitions (or signatures) are added to the antivirus databases. Some antivirus researchers add approximately 200 new records to the database every day. The ability of your antivirus program to detect these new threats depends on the timely installation of virus definition updates on a regular basis. Some updates even include enhancements to the antivirus engine software itself.
You should make it a habit to glance at your security software update status daily, to ensure your security is enabled and up-to-date.