Here is an image from Shigley's Mechanical Engineering Design about circularity... Now here is another image from the same textbook about cylindricity... If circularity applies along the entire length of the cylinder then they seem like they achieve the exact same thing. What am I missing? What is a situation where these two would not be the same?
I'm going to present a highly contrived example of a shape which would conform to the circularity constraint, and not cylindricity:
You can see how in the front view, every cross section is a perfect circle, but it's clearly not a cylinder.
Examples and/or discussion of when you would choose one over the other are out of scope for this question, but suffice to say that circularity is the less limiting of the two, so is used in most cases where cylindricity is not a design-critical additional requirement.
Often the straightness of the cylinder will be controlled by other dimensional constraints or tolerances, making the use of cylindricity redundant, and/or overconstraining.