Viruses are programs that inject malicious code into other applications installed on your computer. This process is called infection.  

The main task of viruses is spreading the malicious code. While infecting files, viruses can corrupt or delete files or even the operating system. Also, they can change the folder structure and block the computer.

Worms are malicious programs that spread through network resources. They are called this way for their ability to “crawl” from one computer to another using information channels.  

Worms spread very quickly. They gain access to the computer, find network addresses of other computers and send their copies to these computers. Worms can use data from address books or mail clients.  

Worms usually create files on system drives but they rarely use any computer resources except for RAM.

Worms spread quicker than viruses.

Trojans are programs that perform unauthorized actions on infected computers. They can delete information from drives, make the system stop responding, steal personal information, etc.  

Trojans do not infect other files. They cannot gain access to computers by themselves and are sent disguised as useful files or programs by criminals. Trojans can cause more harm than regular viruses.

Spyware is software which collects information about a person or company without their consent. Spyware is installed secretly. The task of spyware is:

• Tracking user’s actions.
• Collecting information about the contents of the hard drive. In this case, spyware scans folders and registry to form the list of software installed on the computer.
• Collecting information about connections, networks, router characteristics, etc.

This type of malware poses serious risk on security. Some spyware (e.g. Gator and eZula) allowed criminals control infected computers remotely apart from collecting information.   

Other kinds of spyware are injected into the browser and redirect traffic. If you are redirected to a different page when you try to open a website, then your computer may be infected with spyware.

Phishing is a type of online fraud. Its goal is acquiring access to logins and passwords.  

To make a user enter the credentials, criminals create a copy of a popular online service or banking page and sends a message which is disguised as a message from that service. The message usually contains a request to follow the link and change the credentials. By doing this, criminals make users enter their credentials on fake websites.

For more information about phishing, see the Kaspersky Lab’s virus encyclopedia. To learn how to protect your computer from spam and phishing, see the Kaspersky Lab website.

Rootkits are tools used for hiding malicious activity on the computer. They disguise malware so that it is not detected during antivirus scans.  

Rootkits can also modify the operating system and change its functions to hide its presence and malicious actions performed on the computer.

Cryptors are programs that encrypt important files on the computer, such as documents, images, databases, applications, etc., making them impossible to open. The user is unable to use those files or the whole system. For decryption of the files, criminals demand ransom.

Miners are programs which use the computer for mining without user’s consent.  The computer is connected to the network and its resources are used for mining cryptocurrencies.  

Usually miners get to the computer with the help of a “dropper” app which secretly installs software. Droppers are often disguised as pirated versions of legal applications or activation code generators. Users search for such apps and download them intentionally.

Hoax programs show notifications on fake threats to the user. The main goal of such programs is to make users pay for fake services. The do not cause any harm to the computer but show notifications about a virus attack or computer malfunction. In other words, they inform users about non-existent threats. 

For example, some hoax programs show notifications about multiple registry errors, obsolete drivers, etc. The goal of hoax programs is to get paid for fixing fake errors and eliminating non-existent threats.

Spam is mass mail with unwanted contents. They can contain harmless promos and ads.  

A more dangerous type of spam includes:

• Offers to cash out a sum of money
• Offers to join financial pyramids
• Messages intended for stealing passwords and bank card credentials
• Chain letters 

Spam induces substantial load on mail servers and enhances the risks of data loss.

Programs designed for creation of malware, organization of DoS attacks and hacking other computers. This category includes hack tools, virus constructors, etc.