So, you might think, "Hey, you're a guy, why should you explain anything about periods?"
That's partially true. I'm a transsexual guy. So I know plenty about periods, and I think it's time someone put it straightforward to men.

Several months ago I was in a Sociology class in which one guy, representing for a group of men, asked, "Is PMS real?"

And the reply was a groan from many disgruntled women.

So here it is guys; bluntly explained just for you. Let's forget the hormones and such, and just bring it down to the basics.

Imagine you're sitting in a room, hanging out, being cool or dorky or whatever you usually are. You look down. Your pants are red.
On what planet is that a good day? Multiply it if you're in company.

Women have different problems. Some women get horrendous cramps, especially if they don't drink enough water. A lot of women will feel bloated. Some women have problems with bleeding too much, but a lot have a certain 'type' of cycle they get used to.

Ok, so you have it under control. You either have a half-diaper on, or something shoved up inside of you. Or maybe you're using one of those other more uncomfortable methods. Not a deal-breaker, but not much fun, either.
Let me tell you something about how this works. You don't feel it. It has a mind of it's own. It might be drizzle one day and downpour the next. If it's real bad you might be able to feel it, which will bring in the, "I have to get to a bathroom right now to change".
Mistakes can happen even when you're awake, so if you're having a bad day you definitely don't want to wear any clothing that you want to save.
And then, there's night. That's right. Eight hours of sleep with a leaky faucet. Let's even forget that this is blood. It's not like a stab wound. It doesn't matter too much that it is blood in and of itself (other than it stains); that's just what it happens to be. Let's pretend that you have a carton of dyed water. There's a hole in that carton somewhere, and you can't control how fast water pours out, but normally you can get something to absorb it.
Normally this isn't all that bad. Maybe you put on a bit of 'extra padding' before you go to sleep. But remember, this thing has a mind of its own. You're sleeping one night, and someone decides to come in with a shotgun and blow the carton to bits. You wake up, still not feeling anything and take a look around.
All you can do when you look at the ceiling is wonder, "How the hell did it get up there?" There's no trail or signs of movement. It's just there. And you don't have a clue how it got from Point A to Point B. If you're lucky this won't be much. Maybe you lose a pair of underwear.
If you're not lucky, this can be your underwear, pants/shirt, bedsheets and various other things.

Then there's happy things like swimming. Everyone wants to go swimming, and as they giggle and ask, "Hey, why don't you come in?" you think to yourself, "I can kill you with my eyes".
Ever wonder what things like bicycling feel like with a wad of cotton pushed up against your behind/inside you? Don't.
And those times when you run out of such wonderful products? Welcome to nasty toilet paper land. Make sure you pack extras when you're going somewhere!

Wearing a pad is like wearing underwear with extra starch. I say that because they tend to have a strip, much like tape, to place it on the underwear (sometimes with wings!). It's bendable, but it's not like clothing. If they wrinkle or move around, there's something wrong. If there aren't any wings, it's easy for them to decide that they want to move, because that strip of adhesive isn't all that strong.

And a tampon? Well, in case you guys don't know what the "lost string of doom is", let me explain that. Imagine you go to take out that thing you stuck up inside you, and when you go to pull on the string you can't find it. So now you have something up inside you somewhere, you don't know how far and you can't really feel it. This means you have to reach up inside there and pull it out.
If you can't find it? Welcome to the hospital!

And the hormones? Usually I don't count this much. Hormones can cause a wide series of emotions; and that's usually what they do. That can mean happiness, sadness, anger, irritability, and even feeling extra horny. This string of emotions can throw anyone for a loop, but it doesn't mean anger.
I have a feeling that women have enough reasons to be upset without mentioning the hormones.