File name extension
File name extensions communicate what kind of data a file has and are important for determining which applications can read said file. On GNU/Linux and other Operating Systems this is usually implemented via a suffix to the file name preceded by a period (or dot).
For example, a file called "mystory.txt" has a file name extension of ".txt", which indicates that is is a text file and can be read by applications which can read (and usually write to) text files. See Common GNU/Linux File name extensions for more.