Types of variables

Many types of variables are used to answer research questions

There are several types of variables that can be used in research. First and foremost, variables are chosen to answer research questions.

The PICO mnemonic (population, intervention, comparator, and outcome) identifies important parts of clinical research questions and also helps researchers to think about what variables to choose to answer their research questions.

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Variables

Demographic variables are collected so that samples can be described and so that inferences can be made to other populations as needed.
Demographic Variables
An independent variable is what a researcher manipulates to better understand its association with a dependent variable.
Independent Variables
When a researcher wants to "control" for a variable, this means that they want to hold that variable constant to see if it affects the association between independent and dependent variables. The way to do this statistically is to put all variables of interest into a simultaneous regression model.
Control Variables
A dependent variable is what a researcher measures for to establish an association with an independent variable.
Dependent Variables
Predictor variables are the same as independent variables above. This terminology is often used in applied empirical and clinical settings. A predictor variable is hypothesized to have some sort of association with an outcome variable.
Predictor Variables
Confounding variables adjust the values of a dependent or outcome variable that is associated with an independent variable. 
Confounding Variables
Outcome variables are the same as dependent variables above. Outcomes are always a central focus in applied empirical and clinical settings. The outcome variable should be measured at the current "gold standard" that exists.
Outcome Variables
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There are several types of variables: Demographic variables, independent variables, control variables, dependent variables, predictor variables, confounding variables, and outcome variables.