A selective fire weapon can be fired in either of at least two modes, depending on the position of the selector switch.
All selective fire weapons have a semi-automatic mode, where the weapon automatically reloads the chamber after each round is fired, but requires that the trigger be pulled again before firing the next round. This allows for rapid and (in theory) aimed fire.
Some selective fire weapons offer a burst mode as the second option, where each pull of the trigger automatically fires a predetermined number of rounds, but won't fire any more until the trigger is pulled again. The current U.S. standard assault rifle, the M16A2, fires three rounds with each pull of the trigger when it is in burst mode. The Heckler und Koch MP5 submachine gun (widely used by SWAT teams and Special Operations military personnel) has the capability to fire two to four round bursts, or to be fired as fully-automatic.
The majority of selective fire weapons have a fully-automatic mode as the second option. With each pull of the trigger, the weapon continues to load and fire rounds until the trigger is released or the ammunition is depleted. The M14A1 and M16A1 of Vietnam fame; the ubiquitous Soviet-designed AK47; the Israeli Galil; the FN FAL; and the M4A1 carbine are but a few examples of this type of selective fire weapon.