Spyware is a type of computer program that can spy data you save in disk or activity you do on you computer. Generally, spyware has various destinations. Some monitor everything in relation to Internet; others track all your computer activity such as keystroke. The information recorded by spyware is typically stored in a data file that is then transmitted to the company or institution behind the spyware where it can be utilized for any possible purposes. Some kinds of spyware also process or analyze the data file on your computer to decrease the workload of central servers. The machine that has run spyware is called victim computer because not only secret is exposed but also related resource is occupied by spyware.

Rigorously, spyware should be is a system but not an alone program. In most cases, it works under client/server model. That is, in addition to running on victim computers, a corresponding server, either simple or complex, should accept data from victim computers. Occasionally, the program in victim computer might play the role of server if the spyware is designed for individuals. No matter that it acts as client or server, only the program in victim computer does the job as spy whereas the program in other side does normal data processing. Therefore, we only call program in victim computer as spyware because the program in other side has no characteristics of spyware. When we talk about spyware that runs on victim computer, never forget it usually coordinates with another program in another side.

The nature of spyware is to spy information of victim computer. Nevertheless, no spyware can do everything. For a certain spyware, something it does, and something does not. Its functionality depends on its purposes. Spyware is probably interested in various kinds of information. We list some of the commonest items as follows.

• Which web sites you visit
• What you look at on the site
• Whom you send email to
• What is in the email you send
• What you download from a site
• The files and folders of your computer
• The screen or desktop of your computer
• What you strike on keyboard
• Your ID or password for all Internet activities

The common way to categorize spyware relies on its behavior. Typically, spyware just monitors victim computer and should hide its existing. We call this type of spyware as pure monitoring spyware. So in most cases, spyware is called monitoring software.However, some kinds of spyware need to intervene the operations of user or activities of computer, and users can observe these actions. That is, users will feel there is a program that they even do not know running on their computer. Obviously, the type of spyware needs different ways to implement, and we call them monitoring and acting spyware. Adware and filtering software are in this kind. We list some features of these two categories of spyware as follows. In other words, adware is a type of spyware.

Pure monitoring spyware is designed to collect information. This type of spyware usually emphases to disguise from uncovering and must have network functionalities to send out data. Because all its jobs are done underground, users have little chance or clue to find it unless they are especially smart or have actual experience and knowledge of spyware. In view of developers, pure monitoring spyware is easy to hide and survive since it commonly attracts little attention of users.

Monitoring and acting spyware is designed to both collect information and do necessary operations to intervene victim computer. Sometimes, we hope spyware do something as well as watch something. As long as it has any action such as terminating URL of user or displaying ads in desktop, users will have chance to catch it. Therefore, the real challenge of this type of spyware is how to hide from uncovering and prevent from deleting. For this reason, the monitoring acting spyware generally involves more complicated technology. As we mentioned before adware is monitoring acting spyware.