A quantity is continuous if it can have any value in a range on the number line. Most things we measure in the world — like weight, distance, and time — are continuous.
A quantity is discrete if it can have a limited set of values, like integers or categories. Exact counts are discrete, as well as categorical variables.
A function that shows how density (not probability) is spread across the values of a continuous variable. The area under the PDF within an interval gives the probability that the variable falls in that interval range.
