Any well defined set (group) of objects about which a statistical enquiry is being made is called a population or universe.
The total number of objects (individuals or members) in a population is known as the size of the population which may be finite or infinite.
The population can refer to things as well as people.
All members of the cultural society of your city.
All students of mathematics of Ithaca college.
All Americans who saw 'TITANIC' last year.
Heights of all students of your school.
Weights of all the citizens of city of New York above 20 years of age.
Mileages of automobiles tyres of Dunlop. etc.
A population is finite if it contains finite numbers
of individuals. For example, the ages of 20 boys of your class.
A population is infinite if it contains infinite
number of individuals. For example, the pressures at various points
in the atmosphere.
Often, statisticians want to know things about
population, but they fail to do so almost because in every case
such data for every individual of the population is not available.
Suppose I am a researcher in the field of 'Tuberculosis' (TB). I
want to learn how many Indians suffer from it. It would not be practical
(or perhaps even impossible) to contact every Indian. Thus whenever
we want to study the characteristic of a certain population, it
is difficult to study the whole population. it is often expensive
and time consuming and many times we lack resources for the study
of the whole population. In any science we cannot study more than
a part of population. A part or small section selected from the
population is called a sample.