A few simple words to know for understanding public opinion are:

Biased Sample- people who respond are picked.

Universe- Studied in a poll or survey.

Representative Sample- small group that is a part of the universe.

Random Sampling- everyone has an equal chance of being picked.

Sampling Error- Differ from the universe.

Cluster Sample- Organizes people by divisions and parts of the state.

United States Government: Democracy in Action. Pages 519-522