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.