A spam filter is a piece of software that filters incoming email to separate unwanted email–junk email commonly referred to as spam–and normal, desirable email. Spam is something that no one wants because, while it is annoying, it can also hold viruses, malware and other programs that can do considerable damage to one's computer. Therefore, a spam filter is an important asset to the computer's longevity.

A spam filter works by comparing certain details of an email to a long list of rules. For example, it checks out the title of the email and compares it to a list of rules that would suggest pornographic, pharmaceutical or other malicious content. Another example is the IP address. The filter checks the IP address for possible known spammers and can redirect the email to a specific folder for storage.

The problem with this is that many spammers have personalized spam email by adding things such as "Hi!" or "About our recent conversation." By adding this personalized touch, the spam filter doesn't pick up on the spam and it can get through. To combat this, people have begun adding a second layer of spam filters to prevent that personalized junk mail from getting into your inbox.

One of the growing trends of spam filters is the ability to learn. What this means is that the spam filter can learn what is and is not spam based on your actions. For example, if an email hits your inbox and it's spam, you can hit spam and the filter will update its rule to recognize that type of email as spam. Or, if you are going through your spam box and you notice something that shouldn't be there, if you click "not spam," the filter will update it rules to note that its not a spam type of email.

A feature to look for when using a spam filter is a "white list." This is a feature on a spam filter that allows you to pick what sites are good sites to receive email from. While the spam filter will still sort your email for you, if you add a website to a white list, it means that all email from that site is not spam. For example, CNN send out a free online newsletter. It would normally hit your spam box, but if you add it to the white list, you'll get it in your inbox instead. This gives you control over what sites can get into your inbox and what sites can't. Also, it helps when doing online banking because if you add email from your bank to your white list, you'll know when someone is phishing because if an email comes from your bank and appears in your spam, it's not actually your bank.