File extension is suffix added to the end of filename, which indicates the type of file. Extensions, depending on file system, usually are delimited from the filename with period. Most extensions are three characters long, but it can be from one to more than twenty characters long.
Extensions may be considered as some type of metadata, implying the type of data that is stored in file. Originally, file extension was used to determine file type. The need to condense the file type to three characters often lead to abbreviated/shortened extensions. Examples of that situations are extensions as txt for text, mus for music and others. However, many different softwares started to handle same file types in many different ways, so file extensions started to become closely associated with certain products.
Depending on operating system, filename extensions length can be variable, even some systems alowing multiple extensions on filename. Example, in Unix there are files that contains two extensions, .tar.gz (tar means that file is tar archive of one or more files, gz means that archive is compressed with gzip application.)
Since there are thousands of different file types, there are also thousands of extensions and each can be associated with one or more applications.
txt - Plain text file
html - HyperText Markup Language file
ttf - True Type Font file
bak - Backup file
sys - Windows System file
docx - Microsoft Word Document file
wmv - Windows Media Video file
rtf - Rich Text Format file
bin - Binary disk image file
dmg - macOS X disk image file
csv - Comma separated value file
mdb - Microsoft Access database file