If a research project is set up so that everybody in the population of interest has an equal chance of being included in the sample, the research involves:

Probability sampling exists when every member of the population has an equal, known chance of selection.

This random basis lets the researcher estimate sampling error and generalise confidently.

Quota sampling fills category targets by non-random convenient choice.

Judgmental sampling relies on the researcher's expert selection of units.

Convenience sampling draws whoever is easily available.

Simple random sampling is the basic form of probability sampling.