Cohort Studies

A Cohort Study is a study in which subjects who presently have a certain condition and/or receive a particular treatment are followed over time and compared with another group who are not affected by the condition under investigation.

For research purposes, a cohort is any group of individuals who are linked in some way or who have experienced the same significant life event within a given period.

There are many kinds of cohorts, including birth (for example, all those who born between 1970 and 1975) disease, education, employment, family formation, etc.
Any study in which there are measures of some characteristic of one or more cohorts at two or more points in time is cohort analysis. [Social Research Methods]

Cohort studies are generally prospective; it means such studies involve identifying subjects based on the level of their explanatory variable, and obtaining the corresponding response outcome.
But they could be retrospective too.

These studies usually involve following the subjects over a period of time to determine their outcome.

For instance, many studies have been conducted to compare the rates of breast cancer in women with breast implants and women without breast implants.
Women were identified as either having breast implants or not (explanatory variable), and were followed over time to see whether or not they were diagnosed with breast cancer (response).

Cohort studies are common when it is unethical to assign a condition (such as smoking or breast implants) to subjects, but it is possible to identify existing populations of such subjects.

Cohort studies have to have enormous sample sizes when the outcome of interest is rare in the population.