Collection of information from the representative units in the sample is known as:
Census is the collection of information from all individuals in a population.
Sampling is the process or technique of selecting a sample of appropriate characteristics and adequate size.
Advantages of sampling

It reduces the cost of the investigation, the time required and the number of personnel involved.
It allows a thorough investigation of the units of observation.
A sample can be covered more adequately and in more depth than can a total population.

Reference: Essentials of Preventive and Community Dentistry, Soben Peter, 4th ed page no 288
Census
Sampling
Counting
All of the above