Every member of the target population has an equal chance of becoming a participant. All people's names in the target population are collected then a certain amount of participants are randomly picked (like names out a hat).
People who you just happen to get hold of become participants. This could be done by going up to people on the street and asking if they want to participate.
The target population is divided into different strata. The amount of people in each strata is counted then participants are chosen to be in the sample in a way that represents how their strata appears in the target population.