depends
some have 3.5mm Aux Inputs that are generic
some have a cable that you can buy for your ipod that enables you to change tracks from the steering wheel, have track and artist name etc.. on the head unit.
Just depends on the car.
Got anything specific in mind ?
If you don't own an Ipod and want a 3.5mm headphone socket, they are usually called AUX inputs when listed in the specs.