In quantitative measurements, variables can be discrete or continuous.

The value of a discrete variable is measured in large, discrete units. If you were measuring the length of a train, you might use a tape measure. If you did, it would be possible to make finer and finer distinctions in assessing the length. The possible range of values would included any point between the minimum and the maximum. That would be a continuous variable.

Some visual displays are more appropriate than others for continuous variables. For instance, a frequency polygon makes more sense for continuous variables, whereas a histogram makes more sense for discrete variables.