The way I understand it, is that as long as you have a weapon free (you could swing it if you needed to) in the proper hand (you have sufficient profs assigned to the hand that is holding said weapon) and be within the distance of your weapon's reach (you must be able to at least touch the person with your weapon):
1) You may Parry / Riposte any normal weapon attack for yourself someone else.
2) You may Parry (not sure about Riposte) any Spellstrike for someone else, but you must take the effect as though it had hit you.
2b) You would then be able to call a suitable spell defense (for Parry, not sure about Riposte; probably the same sort of thing--you'd take the effect as would whomever did the spellstrike?) if you had one (spell parry, for example, if it worked; remember, battle magic only for that one); thus you'd be using two defenses to negate a spellstrike on someone else.
3) You may not Parry / Riposte Waylay, for yourself or anyone else. (I hate Waylay, by the way, I think it's a cheesy skill, especially since one is unable to negate it with "fighter" skills.)
4) You may not Spell Parry for anyone but yourself.
If I have any of those points incorrect, please feel free to correct me.