The vagina is a black morass of confusion. You can be a cyborg gynecologist and not fully understand that thing. If we had no standards on Cracked, I'd go into greater detail about the weird things I've seen on vagina-centric websites, but instead I'll say that, beyond the biology of the thing, it's really as hard to get a hold of as a unicorn.
Women menstruate. I learned this on a bus, you may have learned it elsewhere. That's fine. Ladies also require a method of dealing with this, and the result is the one-two sanitation punch of pads and tampons. Go, ladies! That said, I don't think any man, anywhere, fully understands pads. In fact, I'm willing to bet a lot of women don't. Have you seen the pad aisle in a store? It's fucking huge. How does that come to pass? How is there more than one kind? What the hell is the Poise pad that Kirstie Alley is trying to sell me?
The limit of my understanding, having been sent to purchase pads in the past -- if I were a more dishonest person, I would say it was for my beautiful fiery red-headed girlfriend, but I will concede it was actually for my mom -- is that there are a handful of different types that boil down to small, medium and large. Like drink sizes at Taco Bell, this is based on fluid retention. The big ones, you see, are for overnight, because when you sleep you're horizontal and the fluid levels even out, and tidal forces plus sphagnum cause excessive leakage. Or whatever. Medium are for business ladies and small are for chicks who have stuff written on their asses that we're not unsettled to read.
The reason men don't understand pads is pretty simple and can best be exemplified with the following graphic: