The term adware refers to software that displays advertisements and is given to the user embedded into another application.
Adware is considered a legitimate alternative offered to consumers who do not wish to pay for software. There are many ad-supported programs, games or utilities that are distributed as adware (or freeware). Today there is a growing number of software developers who offer their goods as "sponsored" freeware (adware) until the user pays to register.
In the case of legitimate adware, when the user stops running the software, the ads should disappear, and the user always has the option of disabling the ads by purchasing a registration key.
Spyware is a general term used to describe software installed via the Internet on a computer without the consent of the user that performs certain behaviours such as advertising, obtaining information about her browsing habits or changing the computer configuration.
The gathered information can be sent though Internet to a server somewhere, normally as a hidden side effect of using a program and it may be collected for different purposes. Typical tactics include delivery of unsolicited pop-up advertisements, theft of personal information (including passwords to online accounts or financial information such as credit card numbers), monitoring of Web-browsing activity for marketing purposes, and routing of HTTP requests to advertising sites
Spyware may be installed along with other software or as the result of a virus infection. In some infections, its presence is hidden from the user; sometimes are designed to be difficult not only to remove but also to detect. Other kinds of spyware make changes to the computer that can be annoying and can cause this computer slow down or crash.
Users frequently notice unwanted behaviour and degradation of system performance. A spyware infestation can create significant unwanted CPU activity, disk usage, and network traffic.
The anti-spyware programs can work by providing real-time protection or scanning on a regular schedule. In the first case, they scan all incoming network data for spyware and block any threats in a manner similar to that of antivirus. In the second case they can be used solely for detection and removal of spyware software that has already been installed into the computer.