Computer security software can protect you from malware, adware, trojans, ransomware, phishing, and spyware/monitoring software, as well as other common threats.
Malware - is a broad term for any software that will damage your computer or files.
Adware - is one of the more annoying but less dangerous computer security breaches. The goal of this software is to show unwanted ads so the person who installed the adware can make money.
Trojans - are viruses that come bundled with desired files (from movie downloads to software downloads) from less trustworthy sites. They are called trojans and the user lets them into the computer thinking they are safe.
Ransomware - locks a user out of their computers or files until they pay a certain amount or take a certain action, much like a kidnapper holds a person hostage for ransom money.
Phishing - Tries to trick a user into giving up sensitive data like passwords, by pretending to look like a legitimate software or website.
Spyware - Runs in the background of a computer and spies on the user reporting back sensitive data to the hacker.