What's the application extension? (aka .exe, .app, etc.)

In general, in Linux, and so in Ubuntu, the applications do not have extensions. Some examples: nautilus, firefox, gnome-terminal, and so on.

Applications are usually located in these directories /usr/local/sbin, /usr/local/bin, /usr/sbin, /usr/bin, /sbin, /bin, /usr/games, /usr/local/games and others.

You can determine whether a file can be an application if and only if that file is executable. Use ls -l filename, or stat filename, or file filename to determine this. If that filename have this permissions: -rwxr-xr-x, then that file is sure executable.

^{(source of the image: http://www.csit.parkland.edu/~smauney/csc128/fig_permissions.jpg)}