28. What is a web page? What is its main function?

A web page is the representation of a document which resides at a remote site. This is key to understanding what the web itself is and the implications. If a "web" document resides on the machine you are working from, the document is called an HTML document.

A web page has the wonderful feature of allowing embeded references to other pages.

When you "visit" this web page you are instructing your browser to go get a copy of of this page from Virginia. The computer then uses the telephone wires to request a copy from the machine in Virginia. The machine in Virginia sends back the copy and viola, there it is on your screen. If the page were to be modified this evening and you came back tomorrow you would see the new web page (ignore caches for now).

The primary purpose of a web browser is to bring information resources to the user. This process begins when the user inputs a Uniform Resource Identifier (URI), for example http://en.wikipedia.org/, into the browser. The prefix of the URI determines how the URI will be interpreted. The most commonly used kind of URI starts with http: and identifies a resource to be retrieved over the Hypertext Transfer Protocol (HTTP). Many browsers also support a variety of other prefixes, such as https: for HTTPS, ftp: for the File Transfer Protocol, and file: for local files. Prefixes that the web browser cannot directly handle are often handed off to another application entirely. For example, mailto: URIs are usually passed to the user's default e-mail application, and news: URIs are passed to the user's default newsgroup reader.

In the case of http, https, file, and others, once the resource has been retrieved the web browser will display it. HTML is passed to the browser's layout engine to be transformed from markup to an interactive document. Aside from HTML, web browsers can generally display any kind of content that can be part of a web page. Most browsers can display images, audio, video, and XML files, and often have plug-ins to support Flash applications and Java applets. Upon encountering a file of an unsupported type or a file that is set up to be downloaded rather than displayed, the browser prompts the user to save the file to disk.

Information resources may contain hyperlinks to other information resources. Each link contains the URI of a resource to go to. When a link is clicked, the browser navigates to the resource indicated by the link's target URI, and the process of bringing content to the user begins again.

29. What is a computer virus? What are its major characteristics?

A computer virus is a computer program that can copy itself[1] and infect a computer. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability. A true virus can spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive

Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer

A computer virus has 2 major characteristics: the ability to replicate itself, and the ability to attach itself to another computer file. Every file or program that becomes infected can also act as a virus itself, allowing it to spread to other files and computers. The term "computer virus" is often used incorrectly as a catch-all phrase to include all types of Malware such as Computer Worms, Trojan Horses, Spyware, Adware, and Rootkits - all of which are slightly different than Computer Viruses.

30. What is the difference between a virus, a worm and a Trojan horse?

Trojan horse- It is a program, using this program it will make the victim system to make listen on particular port, so that attacker can do anything on your system, i.e. he can tamper with, steal, and destroy the data etc...., a Trojan horse wont spread in to the system like a virus so it won't effect the system performance.

virus- It is a malicious program, using this program it will damage your system, by injecting the virus in to another programs are files, so that it will regrade your system performance. Virus will come to the system with user interaction only.

worm- It is also one type of virus, using this program it will damage your system like virus, not only system it will spread through out the network and checks for honey pot of your os, and enter into the os, it will regrade your system performance and also it will eat your network bandwidth.

31. What are spreadsheet applications? What are their main functions?

Spreadsheet applications (sometimes referred to simply as spreadsheets) are computer programs that let you create and manipulate spreadsheets electronically. In a spreadsheet application, each value sits in a cell. You can define what type of data is in each cell and how different cells depend on one another. The relationships between cells are called formulas, and the names of the cells are called labels.

Spreadsheets generally have a large number of integrated functions for processing data.

There are hundreds, categorised as follows:

Arithmetic functions that offer basic tools to process numerical data

Statistical functions that have analysis tools, averaging tools

Date functions that process and convert dates

Logic functions that process logic data (AND, OR, etc.)

Financial functions that process monetary data