Filefacts.net site contains a database where you can search for file extensions and see detailed information about them. For every file extension you can find information regarding the type of file, and possible software applications that you can use to open such file types.
If you received a file from someone with an unknown file extension that you don't know how to open it, then you can search for that file extension at Filefacts.net, simply use the search box at above to do that.
You may also browse alphabeticically for file extensions using the letters on the top bar, or browse by file type using the categories to the left.
A filename extension is a suffix to the name of a file applied to indicate what kind of file it is. In some operating systems (for example UNIX) a file extension is optional, while in other operating systems (such as Windows) it is a requirement to identify the file type, and allow the operating system to know the software application that will be used to open it.