Why do people put "extensions" after their file names?

On the Mac, it is not necessary to put any extensions (i.e. ".indd") after your file names since the Mac does not use them for identification purposes. However, if you have a Windows machine at home, you MUST use an extension otherwise Windows will not recognize what kind of file it is.

Extensions are also important for any file that becomes part of a web site. Many Mac users add extensions after file names anyway to help identify the type of file it is. For example, a typical Photoshop file might carry the following extensions: "myfile.psd", "myfile.tif", "myfile.jpg", "myfile.gif". All of them might contain the same picture, but each file format has a different purpose.

Hint: Just because a file has an ".eps" extension on it does not automatically make it an Illustrator file. Photoshop also makes ".eps" files but the format of the file is completely different than an Illustrator-created ".eps". Adobe apologizes for any confusion this has caused.