When explaining to children about strangers,one must inform your children that a stranger is someone that they don't know. Also let them know that a stranger can be anybody not just a scary looking person.
My daughter 2 year old daughter thought that only men were strangers, that woman were not strangers lol.
These DVD'S: The Safe Side Combo: Stranger Safety & Internet Safety
are great at explaining about strangers. The kids learn something while being entertained. My children loved this dvd.