Constructors???

[eluser]Tom Taila[/eluser]
Can anyone tell me what constructors are actually used for? the descriptions im getting of many different websites arent getting through to me so if anyone could put it in simple terms or give a half decent example i would be very grateful.
i keep seeing:

parent:: Controller();

or

parent::__construct();

everywhere but i dunno what it is or does? and is it absolutely nessesary or can you manage without ever using it?