Spyware
Spyware is software that runs on a computer without the explicit permission of its user. It often gathers private information from a users computer and sends this data over the Internet back to the software manufacturer.
Adware
Adware is software that runs on a computer without the owner's consent, much like spyware. However, instead of taking information, it typically runs in the background and displays random or targeted pop-up advertisements. In many cases, this slows the computer down and may also cause software conflicts.
Virus
A computer virus is a computer program that can copy itself and infect a computer without the permission or knowledge of the user. The original may modify the copies or the copies may modify themselves. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or carrying it on a removable medium such as a floppy disk, CD, or USB drive. Additionally, viruses can spread to other computers by infected files. Many personal computers are now connected to the Internet and to local-area networks, facilitating their spread. Viruses may also take advantage of network services such as the World Wide Web, e-mail, and file sharing systems to spread.
Computer worms and Trojan Horses
Viruses are sometimes confused with computer worms and Trojan horses. A worm, however, can spread itself to other computers without needing to be transferred as part of a host. A Trojan horse is a file that appears harmless until executed. In contrast to viruses, Trojan horses do not insert their code into other computer files.
Web browser
A Web browser is a software application that enables a user to display and interact with text, images, and other information typically located on a Web page at a website on the World Wide Web or a local area network. Text and images on a Web page can contain hyperlinks to other Web pages at the same or different website. Web browsers allow a user to quickly and easily access information provided on many Web pages at many websites by traversing these links. Web browsers format HTML information for display, so the appearance of a Web page may differ between browsers.
Web application
In software engineering, a Web application or webapp is an application that is accessed via the web over a network such as the Internet or an intranet.
Web applications have the ability to update and maintain Web applications without distributing and installing software on potentially thousands of client computers is a key reason for their popularity. Web applications are used to implement Webmail, online retail sales, online auctions, wikis, discussion boards, Weblogs and many other functions.
Content above courtesy of Wikipedia