I'll never understand the whole cache/cookies thing. 
Cache is a fancy word used to refer to the files your browser stores on your computer, so it doesn't have to keep downloading them each time you visit a site. From a bandwidth standpoint, this is obviously a good thing. However, due to incorrect server or browser configuration, your browser will use outdated files. Which is what happened here with the banner. Oddly enough, I saw the Valentine's Day one until today, but never cleared my cache.
Cookies are also files stored on your computer. But these are only text files, and they're stored at the request of the site you're visiting. They contain information about you and your visit. Like cache, sometimes this good (ex. you don't have to log in every time you visit Facebook), but sometimes this is bad (ex. some sites track your every move and make "recommendations" based on that).
