The type of wiring in residential homes generally depends on when the home was built, where it was built and if it has been updated. In the older homes, knob and tube type wiring was the norm, replaced later by encased plastic-sheathed wiring.
In most cases the newer plastic-sheathed wiring is copper, insulated with plastic and then wrapped with insulation and another layer of plastic for protection. Breaker boxes are another story; sometimes a well-meaning home owner can inadvertently make things unsafe.
It is always wise to consult a reputable home inspector and even an electrician before you buy. A little homework today can save you a lot of money down the road. More on home wiring is available here.