Web sites commonly contain files.
For example, applications or graphics, for visitors to download. Also, your friends, family, colleagues, and others may email you documents to download as attachments. An online user's experience is punctuated with opportunities to download files of interest.
Whenever you download a file (or element) from a Web site, you are specifically downloading the file and its contents which exist in a particular format or file format 
We will refer to them as file formats for technical purposes.
Be sure to check for viruses before you download any type of file.
Run your computer's diagnostic scan over any file you intend to download to protect yourself from infected files.
Every file you download has a file name. A file name identifies a file and comprises a name and an extension.
The extension is a period followed by a two-, three-, or four-letter code that indicates the file format.
For example, bestsellers.doc shows this file is a Microsoft Word document. Microsoft Word automatically tags its documents .doc.
Extensions are vital. Every extension indicates how the document is to be used and downloaded.
You will learn more about the common file formats you can download in the following SlideShow.
 File format:
Web pages are made up of text, graphics, and multimedia elements that are saved in different groups of file formats. Formats are indicated by an extension after the file name, such as sample.txt or document.pdf.