Nice thread title.
Basically as far as I understand (not read much about it, no real interest) you take a feat and can access special abilities of your mount. Special abilities are listed in the MM, other stuff on page 46 in the DMG.
You and the mount are in the same space, attackers can attack one or the other, mount benefits only with feats, if movement provokes OA chooses one target, if you provoke you are targeted, forced movement can't seperate you from your mount unless you wish to. Knocking the mount prone dismounts, knocking you prone allows you to save.
Mounts have the same initiative as you and uses the same actions as you, a mount attack uses your standard action.