Random sample - choosing a sample so that every member has an equal chance of being chosen (this is the absolute definition of random sampling)
Sample - a portion of a large group is termed as a sample.
Biased sample - choosing a sample from people who are under 15 years old (biased sampling states that all the members of a group are not included in survey)
Estimating from a sample - using the responses of a small group to make conclusions about the large group.(estimation means roughly judging something based on some sample )