In computer systems, an extension typically refers to a file name extension, which is a suffix added to the end of a file name indicating its format. For example, the .docx extension specifies a Microsoft Word document. The extension is important because it tells the computer which application created or can open the file and which icon to use for the file. Most file name extensions are compatible with more than one program that you have installed. There are thousands of file extensions related to different types of software, data, and files. Some common file extensions include .txt for text files, .exe for executable files, and .zip for compressed archive files. The file extension also helps an application recognize a file that it can work with. In addition to file name extensions, the term "extension" can also refer to a plugin or add-on that extends the capabilities of software.