In a random sample, everyone has the same chance of being chosen to participate. For example, you will have a random sample if you pick out every 5th person in the phone book to be in the sample.

A representative sample is chosen from a specific group of people to represent that group. A sample of high school students, for example, or a sample of New Yorker readers. You can pick every 5th high school student to be in a sample to represent high school students.

A biased sample is one that is falsely taken to be typical of a population from which it is drawn. For example going to a group of women and ask the question: are men smart? or going to death row and ask "Is death penalty a good thing?"