In absence of any external force, the body would oscillate indefinitely with a constant amplitude a, then it is called free oscillation.

When a body, being acted by an external periodic force ,oscillates with the frequency of the force, then the oscillation of the body is called forced oscillation.

Damped (or free) oscillation occurs when an object is set to vibrate at its natural frequency while forced oscillation involves the application of a force to keep an object in constant or repetitive motion.

The motion of swinging is an example of forced, damped oscillation.

A more simple form of this is simple harmonic oscillation and can be read about here:

http://en.wikipedia.org/wiki/Simple_harmonic_motion

yes,because we supply power through battery ,battery is the external source.

-- pushing a child on a swing

-- 'rocking' a car that's stuck in the snow

-- unclogging a sink drain with a 'plunger'