In Ubuntu, an application can be started by selecting its icon from the application list (triggered by clicking to the icon at the lower-left corner or pressing the window key).
This list of icons is stored in
/usr/share/applications (for all users) and
~/.local/share/applications (for a specific user). The icon is defined in
Icon/application's starting command can be easily figured out by looking at the associated
.desktop file's content at the