In the comments on the post about lightsaber battles, rejiquar asked if there were good beginning resources for writing and/or drawing believable combat scenes. I offered a partial answer there, but since this is a topic that plays to two of my particular interests - writing and martial arts - I thought I'd throw out some thoughts here. This might become a regular series, if it holds my interest and/or if enough of my readers find it interesting, so feel free to ask questions in the comments.
Obviously, I'm not going to be discussing self-defense in anything other than general terms, here. If you want to learn how to defend yourself, you need to find a competent instructor and spend some time training. Instead, I want to look at some elements of combat theory that might be helpful if you want to write medieval/fantasy fight scenes.
So, for starters, let's look at some of the differences between armed and unarmed combat.
First of all: a weapon, any weapon, is a huge advantage. Melee weapons - anything from a knife to a spear - offer a huge advantage in reach, leverage, and damage. With a weapon, you can hit from farther away; you can hit harder, and the edge of a blade (or even just a piece of wood) will deliver the force better than an empty hand or a foot ever will. Ranged weapons take this principle a step further, allowing you injure an opponent while staying well and truly out of his reach - unless he has a ranged weapon of his own, of course. The ideal situation is to have a weapon with enough reach that you can strike your opponent while he's still too far away to strike you.
The brings up one of the very nice things about using weapons: they offer quite a bit of gender equality. A five-foot-tall woman, facing off in unarmed combat against a man who's six-foot-four and twice her mass, is at a serious disadvantage. He has better reach, more weight, and almost certainly more muscle. He can hit her from farther away, and do a lot more damage. And if they move to a grappling situation, he can use his weight against her, too.
Put knives in their hands, though, and suddenly muscle and weight make a lot less difference. With a blade, it doesn't matter so much how hard you hit; it's much more important if you hit, and where you hit. So now the man still has an advantage in reach, but the difference in muscle now makes very little difference.
Now put swords in their hands, instead of knives. The man in this scenario still has more reach, but with each of them holding three feet of sharpened steel, the difference in reach is relatively slight and unimportant. It's still there, but it doesn't matter anywhere near as much. At this point it's mostly technique: skill versus skill.
Change them to using spears, and now there's essentially no difference in reach or ability to inflict damage. As long as nobody is too weak to use the weapon (which shouldn't be the case for reasonably able-bodied adults), the differences in strength and reach are unimportant, and the difference in weight can actually be a disadvantage for the heavier person. (It makes him less maneuverable.)
This also brings up another point: in unarmed combat, it's possible to absorb a blow from your opponent in order to deliver a more powerful blow of your own. I've heard this described as "sacrificing a pawn to capture a queen". With weapons of any sort, it really doesn't work that way. Weapons do so much more damage (and remember, antibiotics are a very new invention, historically speaking) that this sort of trick essentially doesn't work. Sure, you can take a blow to the leg in order to hit your opponent's head - but with any sort of weapons, you're still in real danger of having your leg disabled, not to mention death from blood loss or infection. At the very least, you've rendered yourself unfit for any further fighting until you recover.
From a fighting and/or self-defense perspective, weapons are basically always an advantage.
Oh, one other thought on armed and unarmed combat: there's more stylistic variety in unarmed combat. Some of that has to do with context. A style developed for men fighting in full plate armor is going to look very different from a style designed to fend off multiple unarmored opponents. But a lot of it is just difference in emphasis and body types.
With weapons, on the other hand, there's a certain sort of parallel evolution. There are only so many ways that a blade of a particular weight, shape, and balance can be used by a human body. So you find that katana techniques (designed for use with, basically, a two-handed saber) also show up, historically, in techniques for the Swiss two-handed saber. It's not because one style borrowed from the other; it's because there are only so many effective ways to use a blade of that approximate size and shape.