Here are some of the basics.
#1: Blades come in a multitude of varieties, but as a rule of thumb the main two subtypes are slashing and stabbing. A slashing blade is used best to parry and then lash out in order to cause large gashes through flesh that cause shock and bleed out to easily kill targets. A stabbing blade on the other hand is made to cause massive internal damage in one blow. Depending on the length of a stabbing blade they're either made to pierce armour (shorter), or be used to keep an enemy at bay through sheer range (longer). There are hybrids however, with the
arming sword being an example of such a blade. (You can generally tell if a blade is intended for slashing or stabbing based on its curvature. A scimitar or katana is a slashing blade, whilst a rapier is a stabbing blade. Blades that are only sharpened on one side also tend to be made more for slashing than stabbing.)
#2: When engaging in swordplay, it's half in the footwork and half in the wrist play.
Princess Bride has a really good dueling scene, choreographed and all, that shows this well. Watch the feet: When they're on the defense they don't jump back, they shuffle. Their feet hardly ever leave the ground save when performing silly acrobatics for show, because if you ever lose bearing on where your feet are, you can be easily tripped and impaled by your opponent. That, and when your feet leave the ground, the enemy can predict where you will land and hit you while you cannot dodge. They're also using rapiers, and as you can see, when they're on the offense and not parrying each other, they're largely thrusting--not slashing. With the wrist they can subtly change the point at which the blade will make contact with the enemy, which Westley uses ultimately to confuse Inigo and win the duel.
#3: A lot of swordplay is also straight up grappling your opponent and striking for their hands. (Ergo why
crossguards were invented.) If you can disarm your opponent, they're helpless, and you can easily kill them or force them to surrender! It also tended to be a weak point on knights, as any metal gloves had to remain flexible enough for the user to retain dexterity in their hands. To grapple, you want to engage in parrying with your opponent, then take the opportune moment to shove their blade up to their chest with your own--essentially locking up their sword arm as if they let go of their blade, they become defenseless. You can then easily grab them and throw them to the ground, or unsheathe a dagger (ex: a Tanto) to repeatedly stab your opponent with your free arm.
You can see it all in play here. Again, watch their feet--they shuffle, and only lunge when going for the killing blow. Whenever they can they lock up their opponent's blade, shoving it up to their chest before grappling them to finish them off.
This guy also explains a lot of this visually, the first part being about how you can use the crossguard not only to protect your hands, but to twist your opponent's blade out of their hand, or thrust with his blade locked into the crossguard to impale his defenseless torso.
It should also be noted that much of swordplay was in locking an opponent's attention to you while a friend could run up to them and stab them from behind. Warfare wasn't nice--knights did not play fair. Speaking of, shields should be noted as disposable tools, they don't absorb nearly as many blows as Hollywood predicts and were mainly used to stop arrows and crossbows from instantly killing you, as well as being a piece of metal (or wood) to brute force shove in your opponent's face to put them off balance so you could finish them with your blade.
It should also be noted that most duels last
seconds. At the most, maybe a minute or two. "He who strikes first, wins."
EDIT
Oh, and it should be noted, rapiers weren't typically used on battlefields. They were specialized to dueling and
nothing else.
