This document gives an overview of computer viruses and other types of malware.
What is malware?
Malware is short for malicious software. It is a term for software that is intended to harm the user in any way.
What is a virus?A computer virus is piece of computer code, usually hidden within a seemingly innocuous program, that replicates itself and inserts those copies into other programs or files. Viruses usually perform one or more malicious actions on the infected system (such as destroying data). Computer viruses never occur "naturally"; they are always man-made. Once created and released, however, their spread is not usually under direct human control.
Macro virusesA macro is a piece of code that can be embedded in a data file. Some word processors (e.g., Microsoft Word) and spreadsheet programs (e.g., Microsoft Excel) allow you to attach macros to the documents they create. In this way, documents can control and customize the behavior of the programs that created them, or even extend the capabilities of that program. For example, a macro attached to a Microsoft Word document might be executed every time the document is saved and cause the text of the document to be run through an external spell checking program.
A macro virus is a malicious macro that is attached to a data file. In most respects, macro viruses are like all other viruses. The main difference is that they are attached to document files rather than executable programs.
WormsWorms are very similar to viruses: they are computer programs that replicate themselves, and they often (but not always) interfere with the normal use of a computer or a program. The difference is that unlike viruses, worms exist as separate entities; they do not embed themselves in other files or programs. Although worms are technically distinct from viruses, they are still commonly referred to as viruses.
TrojansA Trojan Horse is a program that comes with a hidden surprise intended by the programmer, but totally unexpected by the user. Trojan Horses are often designed to cause damage or do something malicious to a system, but are usually disguised as something useful. Unlike viruses, Trojan Horses don't make copies of themselves. Like viruses, they can cause significant damage to a computer. Although Trojan Horses are technically distinct from viruses, they, like worms, are still commonly referred to as viruses.
Rootkits are designed to remotely access and control a machine without being detected. They are called rootkits because they have "root" or full admin privileges on the infected machine. This allows it to change settings, alter security software, run executables, and steal personal files. Although some rootkit detectors do exits (e.g. TDSS Killer), rootkits are difficult to detect and often go unnoticed by antivirus programs.
Adware and Spyware
Adware is a type of malware that delivers advertisements to the users. These programs are sponsored by advertisers and are created to make profit from advertising revenue. Adware programs can also contain other types of malware.
Spyware is software that tracks the user without their knowledge. This can include logging keystrokes, stealing login credentials, and monitoring activity on the user's machine.
A bot is a piece of software used to run automated tasks over the internet. Although many bots are used for harmless purposes, Botnets (or collections of bots) are capable of DDoS attacks and mass distribution of malware.
A bug is a software flaw that produces an undesired outcome. Most often, they are created accidentally by the software developer. Although bugs are not malicious by nature, security bugs can allow hackers to bypass security measures and gain access to secure systems.
Information adapted from Indiana University Knowledge Base, Symantec AntiVirus Research Center (SARC), and Veracode Common Malware Types: Cybersecurity 101.