Cavalry Rules

Impact attacks: Impact attacks are part of movement, so if you have a unit of cavalry you do the impact attacks of each model when you are moving them before you declare the charge target of the next model. Impact attacks do not have to target your charge target and it's often better if they don't, as impact attacks occur during movement if you kill your charge target you will have failed your charge. You keep moving until you run out of movement or run into another model you can't move through (as per the charge rules) and then your activation ends immediately. It can actually be pretty bad if you don't take that into account as you can take a bunch of free strikes or be out of formation. Doesn't matter if you hit a jack or something that's going to survive a charge though. Remember that an impact attack is a thresher with 0.5" range and you only make it if you contact the model. You can also use it to run over something in the way of your charge.

Ride by attacks - you can move shoot or fight and then move again. Really good for things like the Khador and Troll battle engines. It's an order so you can't ride by attack and charge.

Tall in the Saddle - You can ignore intervening bases smaller than your own. Allows you to attack a small based model that is being screened if you have reach.

Cavalry Charge - You get +2 on attack (to hit) rolls with Heavy Cav but not on your impact attack rolls.

Light Cavalry - No impact attacks, no ride by attacks but you can use your mount attack without charging.

Light Cavalry Move - this was errated and the language was confusing (I think the language might have been changed again in subsequent erratas). You get a light Cav move as long as your activation has not ended. So if you ran or failed a charge or did something that caused your activation to end then you cannot make a light cavalry move.

Edit - Ride by attacks and cav charge bonus added