A very simple explanation:
Bandwidth has to do with how much data you're using.
RAM is how much memory it takes up to process the data.
So if you have a very large image, that would take a lot of bandwidth every time people view the page, but very little memory, since it's just showing a picture. On the other hand, if you have a complicated program that has to run processes every time you access a page (like WordPress), that would use more RAM, but probably not as much bandwidth.
(Watch out for when your bandwidth is sky high AND your RAM is... a crash is about to happen.)