I've been talking a lot about exposures and health outcomes, and we also use this term "variable," so just a quick definition - a variable is any measured characteristic that may differ among individuals, so variables can include both the exposures of interest and the health outcomes.
There are many potential exposures that can have an impact on health outcomes and that might be of interest in these epidemiologic studies.
We can categorize them in a logical way according to these four categories: Innate factors that one is simply born with - their sex, their race, and ethnicity -basically their genetic composition. Then there are acute exposures, by which we simply mean those that are relatively brief.
You might be exposed to an infectious agent during a flu epidemic.
For a fetus it might be an intrauterine exposure. It might be a relatively brief physically or mentally stressful events - anything like that.
Chronic exposures refer to things like pollution, but we can also consider social factors such as poverty and even policies and laws as chronic conditions that might have an impact on health.
And then finally there are many exposures that are time-varying. This certainly would apply to our behaviors - how we eat, what we eat, our diet, exercise, whether we smoke or drink alcohol and how much. All of these things that might be changing over the life course, and it may be of interest in some studies to keep track of how those exposures change. We'll also be talking about health outcomes or health indicators. And again, these are really any variable that tries to assess health or disease status. They can be binary, meaning they are factors or outcomes that either occurred or did not occur, e.g., diseased or not, living or dead. That kind of thing. They can be ordinal - meaning simply graded categories, grading say from poor to good to excellent.
Or they can be continuous outcomes - measurement such as systolic blood pressure, serum cholesterol levels, and so forth. But for all these things the key to understanding the determinants of health and disease is recording of these events and exposure status. 
The illustration here shows an old fashioned birth report and a certificate of death.