What are Dark Arts?

[potioncat]

No: HPFGUIDX 91055

I have some questions about Dark Arts and DADA. DADA in the first 
years is more about getting rid of magical pests than fighting dark 
arts.

 After Ron attempts the slug curse on Malfoy, Hermione comments "That 
curse is difficult to do under the best circumstances." (paraphrased 
from CoS) 
Hermione puts a spell on the D.A. list that hexes anyone who betrays 
the group.(OoP)  With all the hexes, curses and jinxes that are being 
performed by fairly young students at Hogwarts, just what separates 
dark magic from regular magic?

We're told in OoP that young Snape was up to his eyeballs in dark 
magic, and that James hated dark magic.(by Sirius)  We're told in PoA 
that the Marauders' map is full of dark magic.(by Snape) Lupin later 
gives it back to Harry, so he must not think it's dark magic.  We 
don't know if Snape was just poking at Lupin, setting up a red 
herring for Harry, or in his opinion it is dark. I'm under the belief 
here that Snape knows who created the map.

Just about any of the magic the students are being taught could be 
used for good or evil.  So, what are Dark Arts?  Does anyone have a 
good feel for this?  Does anyone have a canon reference about this?

Potioncat