As Tester says, Shirlock is referring to a switched neutral. The switch should be stopping the current on the black line from getting to the light when turned off, but there are lots of ways to wire a switch where the wire colors don't help. In particular, if the wiring begins in the light fixture and uses a single pair of wires to the switch, you'll only have a black and white wire in the switch J-box.
To know if you have a switched neutral, you'd need to check if there is any voltage to ground from inside the switched outlet or light socket when the switch is turned off. If it's a light socket and you don't have a ground, you can use the large pin on an extension cord. Just use a voltage tester to see if there's any current, they are cheap and I'd consider it a required tool if you're doing any electrical work.
But Tester is also right that if you had a semi-competent electrician wire your home, and replaced the switch with another and using the same wires, you are unlikely to have this issue. And if you never stick your finger in the light socket, you'll be unlikely to ever realize you had this problem.