Disclaimer: I am NOT a physician. This node should never be used in lieu of seeking proper medical advice and care. It is for informational purposes only and I take no responsibility for what you do with this information! Don't be stupid, if you've punched a wall and are unsure if you've damaged yourself, GO TO AN EMERGENCY ROOM NOW.
What is it?
The Boxer's Fracture, sometimes also known as a Brawler's Fracture (and, whoops, the Boxer's Break), is a break in the fourth or fifth metacarpal bone of the hand. It is called this because it is typically incurred by punching a hard object. The use of Brawler's Fracture emphasizes the fact that trained boxers are much less likely to have this happen to them, as they both wear gloves designed to protect their hands and have been taught to hold their wrists in such a manner as to minimize the chances of this injury.
The metacarpal bones are the bones which connect the fingers to the wrist, and as such form the actual structure of the hand itself. If you look at the back of your hand, the knuckles (which are what hit whatever you've punched, usually) are in fact the heads of the metacarpal bones, or one end of them. The other ends are bunched back near the wristbone.
Why does it happen?
Take the Y Chromosome. Add alcohol. Toss in any spice like loud music or disputes over girls or...oh, sorry, you mean mechanically?
When you make a fist, the fingers curl up and the knuckles themselves are presented prominently. A proper straight punch (I believe) should strike with the knuckles of the index and middle finger, since those fingers are both the furthest extended and are backed by the strongest of the metacarpals. In addition, when the fist is made, those metacarpals are (or should be) straight on to the objective from the fist, meaning any shock taken against those knuckles is transmitted down the long axis of the bone. This is desirable, as it is the least likely to cause injury to the bone itself.
The pinky finger and ring finger, however, are not in the same position. If you look carefully, you can see that the knuckles of those fingers are actually bent 'down' slightly, as the fingers curl under and pull the metacarpal into a slight downward angle (with the fist held out level). As a result, if a misaimed punch strikes a hard object on these two knuckles, the force of the blow will actually hit the head of the metacarpal bone at an angle to the shaft of the bone behind it, not dead on. In essence, the strike will force the fingers to curl further, past their breaking point. If this happens with enough violence, the metacarpal will break - typically, it will crack just behind the head (knuckle) as that part bends down.
A slightly less common but still typical injury is for the metacarpal to crack at the back end - if the fist is rotated down at the wrist, slightly, but the knuckle strikes more straight on. This will exert force straight(er) through the knuckle, but at an angle to the back of the metacarpal where it joins the wrist, and the crack will occur at that end.
How can I tell if I have one?
Unlike many broken bones, the Boxer's Fracture doesn't produce obvious structural effects, since it's near a joint and not in the midst of a long bone. In addition, the other metacarpals will ensure the hand itself holds shape. What will happen is that the break will hurt like blazes. This isn't usually enough to make people seek attention, because of course they've just punched something like a wall (hard) and expect their hands to hurt. The thing to look for is swelling; a Boxer's Fracture will produce a severe local swelling around the break itself. Especially if the break is near the back of the hand, at the wrist, a swelling is indicative of a fracture.
Of course, if the pain after hitting something doesn't go away after a minute or two, seek help. Many people actually wake up with a hangover and a Boxer's Fracture. Imagine that.
What Should I do?
First of all, again, see a doctor. On the way, though, ice the hand, immobilize and elevate it. This will reduce the swelling and the pain, and immobilizing the hand reduces the chance of exacerbating the break or doing further damage. Your doctor will determine what needs to be done. Sometimes the best thing to do is simply splint the hand to prevent its use and allow the break to heal; casts are almost never needed.
What will happen to my hand?
Assuming you realize it's a break and see a doctor relatively quickly, probably nothing long term. The bones of the hand evolved to deal with damage. If the hand isn't properly immobilized, the fracture can sometimes heal with the metacarpal bent slightly in the direction of the break, which will result in the hand having a noticeable angle behind the knuckle that was affected. In extreme cases, this may have an impact on range of motion and increase the risk of tendonitis in nearby joints.
Painful personal experience, gleeful high school buddies who are now orthopedic surgeons, and helpful ER technicians.