Coverage errors occur when the sampling frame excludes some segments of the target population. Phone books are very convenient, but unlisted households are excluded. Election polls sample from the frame of registered voters, but the target population is the subset who are going to vote.
Nonresponse errors can cause serious bias in survey results. Phone interviewers find it easiest to reach families with young children, and hardest to reach young singles. Mail surveys tend to be answered by those who feel strongly about an issue, or by those who feel more civic responsibility, neither of which is a representative cross-section of the population.
Measurement Errors occur when respondents answer `inaccurately' because of question wording, question ordering, interviewer effect, or other external influences. For example, the answer to ``Do you approve of affirmative action?'' may be influenced by the gender of the interviewer. Also, the question ``Do you like living in Davis Hall?'' may be influenced by preceding it with the question ``Do you have enough parking spots in Davis Hall?''.