First of all, there are different types of light/medium cavalry.
1)Lancers (polearm builds)
2)Bumpslashers (1h/2h builds)
3)HA (HA)
4)Throwers
5)Xbowers
1)The lancers concentrate on speed and damage by couching enemies. Their primary target is heavy cavalry and archers, and kill them by couching them from the sides (heavy cav) or from behind (archers). They usually have a courser, or another very quick horse, a heavy lance, some small and fast shield, and a secondary weapon. They are agile but fragile, and best stay out of combat. They§ use light armour to quickly couch or lance enemies.
2)The bumpslashers are versatile, because their weapon can be used to fight infatry as well as cavalry. This class requires much timing, but the reward is great. With enougb speed bonus, they can engage heavy cavalry, and kill plated enemies. They are weak VS archers, who tend to shoot them while jumping aside. They usually use a 1h sword and shield or 2h in 1h mode, or a morningstar for crushtrough. The horse I recommand is a destrier, it is quite durable and fast. They have medium to high armour to ensure survivability against archers.
3)I don't know anything about HA.
4)I don't know enough about throwing from horseback, but that you also need horse archer skill
5) The Xbow cavalry uses a crossbow that ca be reloaded from horseback and try to kill other light cavalry with the more devastating shots from their crossbows. Or they use heavy crossbows wich they reload outside combat. Then they have the ability to chase and kill heavy cavalry.
Enemies
Your main enemy as light/medium cavalry is archers. They can kill your horse in up to 3 arrows, while you try to engage them. Try to avoid attacking aware archers, flank them instead. When you are attacking an aware archer, try zigzagging, slowing and accelerating, and randomly moiving to avoid as many arrows as possible. When in melee range, try to bump them, so you have the chance to turn around and engage them. If you miss the bump, try to dodge the arrows he sends after you while trying to escape.
Another dangerous enemy is the crossbow. They can heavily damage your horse, or kill you while charging at them. To fight them, make sure they miss their first bolt, and then engage them while they are reloading.
Pikeman are your nemesis. They can stop your horse from a large range, and let the common cavhate finish the job. The best you can do is avoid them, and when stopped, dismount yourself, so you wont get KD'ed by your horse dying.
Even more dangerous than pikeman, are people with 2h swords. They outrange your lance while couched, and they do heavy damage to your horse and you. Thanks to the lolstab, they can damage you for a long time while driving past, and have a small invisible range.
Another dangerous enemy is throwing. Their heavy weaponry in the hands of a high-level or skilled player can do devastating damage to you and your horse. The best solution is to avoid attacking them when they are aware, and flanking them. Especially watch out for throwing lances, they end your horses life really quickly.
To stay alive, try to stay out of melee mobs, and streets full of archers. When you horse is at low health, try to dismount before you get KD'ed by your horse's death. This will ensure yours too.
Tell me if I have forgotten something, and please comment. I hope this guide was usefull.