Some people may use terms like “virus,” “trojan,” or “worm” interchangeably, but they should not. Each term has its own definition, and they operate in certain ways. For instance, the term “malware” refers to any program that is malicious. Thankfully, antiviruses remove all types, from malware to trojans to worms.
With that said, what are the differences between these three?

Viruses

A virus has one purpose, and that’s to infect files. Think of it as a real virus, which will infect cells. By running a file infected by a virus, it will spread to the files on your system. It typically spreads to the .exe files, so when you run the programs, the virus will run. Viruses infect other file types, including Office documents with macros.

So how do they harm your computer? By doing quite a bit. They may overwrite your program files with themselves instead of attaching to the program, or they may delete your files. Viruses are memory hogs, and this can lead to constant computer crashes. With real viruses, they’re dangerous because they spread. A simple transfer or click can spread the virus everywhere, and you can’t stop it.

Worms

A worm, meanwhile, doesn’t need to be executed to spread. They don’t infect other files, but copy themselves instead. Worms may worms virus trojans-03email themselves to everyone in your address book, such as the Mydoom worm, and they may come in through vulnerabilities in the systems. These are typically not updated and have their firewalls off.
These worms can typically cause a lot of traffic if they transfer through a network, causing immense slowdown. It can cause the same amount of damage as a virus, so they need to be eliminated as soon as possible.

Trojans

A trojan is named after the myth of the trojan horse. You probably are familiar with this story, but if you’re not, here’s a refresher. The Greeks needed to take over the city of Troy, and to do this, they made a giant wooden horse and gave it to the citizens of Troy as a gift. However, inside the horse were Greek soldiers. The horse was taken into the city, and at night, when the city had its defenses down, they came out of the horse and opened the gates, allowing them to attack the city when they least expected it.

So trojans do the same thing. They will pose as software that seems like it has a use, but while you install it, it allows a backdoor to be opened, letting risks in. The person who created the trojan then can use your connection to do horrible activities that can be contributed to you, or install anything they want onto their system. But trojans can only work if you execute them.

Misc. Threats

Here are a few other types of malicious programs.

Spyware-As the name implies, it will spy on your computer. These include keyloggers, which will record every keystroke you make in order to steal your information, or it may include adware that will monitor your browsing and send you personalized ads. These are made to make money, and many antiviruses do not remove them. However, Windows Defender for Windows 7 will do just that.

Scareware/Crimeware-Scareware, well, scares you into downloading their malicious software. It may pop up on a page and claim that you have hundreds of viruses on your computer, and only their antivirus can protect you. Their antivirus will tell you the same thing, and it will say to buy the program to remove it. Once you pay them, the scareware goes away. Of course, you can remove it and not give them a penny.

Overall

No matter what it’s called, these threats pose a danger to your computer. Therefore, you should keep your antivirus up-to-date, use a firewall, and make sure you don’t click on anything suspicious. While it won’t prevent you from getting a malicious program completely, it will greatly reduce the risk.