In order to study the relationships among variables, observational studies are performed. Unlike controlled experimental designs where only certain variables are allowed to vary (at prespecified levels), in observational studies the variables are observed and recorded. Often some of the variables are controlled as much as possible. Consider a long term study on a drug involving humans where a variable that needs to be controlled is diet. The diet guidelines are set but these will probably be broken from time to time (or maybe often) by some of the human subjects. Contrast this with a lab setting, where the diet of animals can be controlled.
In observational studies, cause and effect are hard (often impossible) to establish. But associations and predictabilities among variables can be investigated. Such associations and predictabilities may be further studied in a lab setting.
Source: Porta M, Greenland S, Hernán M, dos Santos Silva I, Last JM, ed. A dictionary of epidemiology, 6th edition. New York: Oxford University Press; 2014.
Everitt BS. Statistical methods for medical investigations. 2nd ed. New York: Halsted Press; 1994.