As you know, software that damages any part of the system (encrypts, deletes, allows you to increase your rights in the system (root, administrator) or retrieves your data) is called a virus. I will tell you about some types of viruses;
1) Worms => It is sent as a website via e-mail with social engineering. If the URL in the e-mail is clicked, a malicious file is automatically downloaded to your computer. After the virus is infected, it starts to copy itself by working in the background (does not make you realize that it is running) and consumes areas such as hard disk capacity, making your computer unusable. In addition to these, these viruses can steal your data from your computer or create a backdoor (i.e. can access your computer whenever it wants). 2) Trojan Horses => Trojan horses, which seem to be good software, are activated when run on the computer. The reason for its name Trojan horse is that the Trojan horse in mythology is sent as a gift, but soldiers come out of it and attack there. It is very difficult to detect these viruses, so you can only notice this virus after a certain period of time when it starts to harm your systems. When you infect your computer with this virus, the attacker can secretly monitor you, delete your files, slow down your internet, and make changes to the files. 3) Spyware => Viruses that infect computers or mobile devices and collect personal data about the user such as internet usage habits, e-mail addresses, bank account information, passwords without the user's knowledge and ensure that this information is transmitted to the attacker. NOTE: Viruses have fixed attack targets, some of them are; Password Stealers, Banking Trojans, Infostealers, Keyloggers 4) Rootkit => In short, they are malicious software that damages the operating system kernel and gives attackers full authority over your computer. Since they work at the operating system kernel level, detection, analysis and deletion processes are difficult. It can remotely control the accounts you use, your bank and credit card information, your camera, your microphone, or it can capture this information. 5)Ransomware =>
It is called malicious software that infects the operating system, renders files unusable and encrypts them. This virus is usually a virus that comes via e-mail and contains a malicious link in its content and is transmitted by clicking on the link. The attacker puts an explanation and an e-mail address to contact the file content. After the attack, contact is made via e-mail, the attackers usually demand a fee in bitcoin and share the password with you when you send the fee. Most attackers keep the files in their own backups, no matter how much they say they will delete them) 6) Remote Administration Tool (RAT) => This virus is one of the most dangerous viruses that gives the attacker unlimited access to the target machine when it is active. Using this virus, the attacker can transfer files, add / delete files and programs, and even take control of the keyboard and mouse.