Oh, er... [It's always a little strange -although not entirely uncommon– to come upon someone who doesn't know about star systems and the unfathomable dark expanse that is space.]

It's the word used to describe the collections of planets and stars associated with a particular galaxy. Galaxies in turn make up the universe, which holds an infinite number of stars, planets and open space in itself.

It's... all quite vast and complex, as you might imagine.