Virus's:

In computer security, a computer virus is a self-replicating computer program that spreads by inserting copies of itself into other executable code or documents. A computer virus behaves in a way similar to a biological virus, which spreads by inserting itself into living cells. Extending the analogy, the insertion of a virus into the program is termed as an "infection", and the infected file, or executable code that is not part of a file, is called a "host". Viruses are one of the several types of malicious software or malware. In common parlance, the term virus is often extended to refer to worms, trojan horses and other sorts of malware; viruses in the narrow sense of the word are less common than they used to be, compared to other forms of malware. However, in a stricter sense, viruses, worms and trojans are different from one another. They have different characteristics and behaviors.

While viruses can be intentionally destructive, for example, by destroying data, many other viruses are benign or merely annoying. Some viruses have a delayed payload, which is sometimes called a bomb. For example, a virus might display a message on a specific day or wait until it has infected a certain number of hosts. A time bomb occurs during a particular date or time, and a logic bomb occurs when the user of a computer takes an action that triggers the bomb. The predominant negative effect of viruses is their uncontrolled self-reproduction, which wastes or overwhelms computer resources.

Today, viruses are somewhat less common than network-borne worms, due to the popularity of the Internet. Anti-virus software, originally designed to protect computers from viruses, has in turn expanded to cover worms and other threats such as spyware, identity theft and adware.

Spyware:

In the field of computing, the term spyware refers to a broad category of malicious software designed to intercept or take partial control of a computer's operation without the informed consent of that machine's owner or legitimate user. While the term taken literally suggests software that surreptitiously monitors the user, it has come to refer more broadly to software that subverts the computer's operation for the benefit of a third party.

In simpler terms, spyware is a type of program that watches what users do with their computer and then sends that information over the internet. Spyware can collect many different types of information about a user. More benign programs can attempt to track what types of websites a user visits and send this information to an advertisement agency. More malicious versions can try to record what a user types to try to intercept passwords or credit card numbers. Yet other versions simply launch popup advertisements.

Maleware:

Malware is software designed to infiltrate or damage a computer system without the owner's informed consent. It is a portmanteau of "malicious" and "software". However, many normal computer users are still unfamiliar with the term, and most never use it. Instead, "(computer) virus" is more commonly used in common parlance and often in the general media to describe all kinds of malware.

Malware is a general term used by computer professionals to mean a variety of forms of hostile computer code. Another term used for this is "badware", but this new term was apparently coined to mean any "software that fundamentally disregards a user's choice over how his or her computer will be used." (http://www.stopbadware.org/home/faq) In that sense, "badware" is a more general term that encompasses "malware", which then means only malicious software.

Software is considered malware based on the intent of the creator rather than any particular features. It includes computer viruses, worms, trojan horses, spyware, adware, and other malicious and unwanted software. In law, malware is sometimes known as a computer contaminant, for instance in the legal codes of California, West Virginia, and several other U.S. states [1].

Malware should not be confused with defective software, that is, software which has a legitimate purpose but contains harmful bugs.

Adware:

Adware is software integrated into or bundled with a program. It is usually seen by the programmer as a way to recover programming development costs, and in some cases it may allow the program to be provided to the user free of charge or at a reduced price. The advertising income may allow or motivate the programmer to continue to write, maintain and upgrade the software product.

Some adware is also shareware, and so the word may be used as term of distinction to differentiate between types of shareware software. What differentiates adware from other shareware is that it is primarily advertising-supported. Users may also be given the option to pay for a "registered" or "licensed" copy to do away with the advertisements.