Marks after completing dungeons

[Xerxes007]

Can anyone explain to me, what the marks at the end of each dungeon mean, and how they affect umm. whatever?

For example, after killing the boss, i always get the marks A S S, what does that mean?
Thanks

[HaxJester]

I have noticed that lower ranking give less exp (A gives less than S), not sure if anyone else have gotten this