Malware, short for malicious software, is software designed to infiltrate a computer system without the owner's informed consent. The expression is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software or program code. The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware, including true viruses.      

A computer virus is a computer program that can copy itself and infect a computer. The term "virus" is also commonly but erroneously used to refer to other types of malware, adware, and spyware programs that do not have the reproductive ability. A true virus can only spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive.   

Adware, or advertising-supported software, is any software package which automatically plays, displays, or downloads advertisements to a computer after the software is installed on it or while the application is being used. Some types of adware are also spyware and can be classified as privacy-invasive software.

Spyware is a type of malware that is installed on computers and collects little bits of information at a time about users without their knowledge. The presence of spyware is typically hidden from the user, and can be difficult to detect. Typically, spyware is secretly installed on the user's personal computer. Sometimes, however, spywares such as keyloggers are installed by the owner of a shared, corporate, or public computer on purpose in order to secretly monitor other users.

Bots (Robots)  applications that run automated tasks over the Internet.  May or may not be malicious, when malicious, they typically take over your computer to use it to attach other computers over the internet.  It may also include a key logger to steel your passwords.

Crapware is unwanted software that is installed on your computer when you buy it or piggybacks on other software you are downloading. For example, when installing Adobe Acrobat Reader, it defaults to also installing McAfee Security Scan Plus unless you specificly uncheck it.

A firewall is a part of a computer system or network that is designed to block unauthorized access while permitting authorized communications. It is a device or set of devices which is configured to permit or deny computer applications based upon a set of rules and other criteria.  Note, typical anti-virus software does not cover this threat unless they are called security suites.  Microsoft has and adequate one included all operating systems from XP on.
Firewalls can be implemented in either hardware or software, or a combination of both.

A rootkit is a software or hardware device designed to gain administrator-level control over a computer system without being detected.

Ransomware is program that wants money to remove some tipe of threat.  One of most well known proprams to be from an organization like the FBI and wants money as a fine.  Others that are close to ransomware are programs that will volunteer to scan for viruses or speed up your PC but then when you get to the end it will list a large number of problems but will not fix them until you pay money.  The list of problems may be highly inflated and include a lot of registry problems that usually do not cause any serious problems.  I have at times fixed over 300 registry problems on a computer and found no change in performance.  I did have a worm feeling in side that I had done something good for my computer so the time was not totally wasted.

Keystroke logging (often called keylogging) is the action of tracking (or logging) the keys struck on a keyboard, typically in a covert manner so that the person using the keyboard is unaware that their actions are being monitored.

Phishing is attempting to get sensitive information such as passwords by pretending to look like a legitimate web site or legitimate e-mail.  For example it might be an e-mail asking you to log on to your bank web site because there has been a problem that you need to fix.  They then include a link to your bank that actually takes you to a false site but one that is made to look like your bank's.  When you log in you will have given them your user name and password to your bank.