Cascading Style Sheets, or CSS for short, are like waterfalls except with style sheets rather than water. They cascade down pages and websites, which are like rocks except that they're on computers. They can make pretty rainbow colours if you get the angle just right, or they can drown you if you fall in. You could also hit yourself on a web page or get eaten by fish.
But what are style sheets? They're like water. Sort of. They're thingies composed of sets of rules which are kind of like molecules or algae or something. Water comes in sheets, except when it comes in drops, puddles or mist, but nobody cares about that because in waterfalls it's sheets or curtains, and maybe drops but whatever. There are also fish and things, kind of like how there are ids, classes and whatnot in CSS, and they eat the planty things and might bite you if you're not careful.
CSS has had multiple editions - 1, 2, 2.1, 3 and 4 - similarly to how waterfalls come in multiple editions. With each subsequent edition, more unicorns have been added. Unicorns from CSS 2 don't work in CSS 1, and so on. The point of updating CSS is to have fun and provide homes for loose unicorns that appear in people's heads.
Not all browsers support CSS because the style sheets are sometimes wet (with coffee, as water is never found inside computers) and the whole thing works by some kind of voodoo magic. However, long about IE 3.0, people realised that computers are just witchcraft anyway and it's either attach them to a log and stick them in the water or support CSS. The latter was more practical or something.
Not enough pregnancy jokes.
Rainbow colours are really shiny.