Every time you save a file, your computer will add an extension to it. Very often, it will ask you with what the extension you would like to save it as. It is important to know at least a few basic types of file extensions, as choosing the right one to save your file with is important. That is because it will inform your computer with what program to open it next time you will want to edit it. Let me here give you a few most basic and common extensions: .doc is a MS Word file, .xls is a MS Excel file, .pub is a MS Publisher file, and .ppt is a MS PowerPoint file. Among the programs used on the Internet, here are some of the most common used ones: .mp3 is a sound or music file, .pdf is a portable document file, .mpg is a movie or animation, .png is photograph or image, .jpg is also a photograph or image, .gif is a graphic or animation and finally .htm is a web page file.
As I said, it is important for you to know at least a few basic extensions in order to be able to save your documents in the correct format. Even if you use the wrong type of extension to save you file, probably nothing that bad will happen, but you may have problems opening the file in the future. You may also forget what data this file contained in the first place. It is better to be careful about that.