File formats and file extensions are often mistakenly described to be interchangeable. Actually, the file format is the way the data in the file is organized, while the file extension is the characters that appear after the period.
Following the last period in a file name, the file extension is a series of characters used to identify the file and locate it in the file system. File extensions are designed to store a variety of different types of data, and they must begin with a period and have at least one letter, but there are no additional formatting requirements for an extension.
Sites with extensive lists of extensions often break them down into categories including; Audio, Video, Text/Word Processing, Spreadsheet, Presentations, Database, Systems, Web and Image. This post explains the 2 basic Image formating options, a few of the more common file format extensions for images and the use of each of them.