What is a RINO?

I have been hearing alot of talk the past few days about taking back the Republican party and getting back to its conservative roots. I have also heard lot of talk about purging the RINO's from the party.

Can anyone tell me what a RINO is? I am well aware of the what the letters stand for but is there a line in the sand from a RINO and a conservative. Is it black and white?

I am under the impression that the main things conservatives stand for is lower taxes, small government and less regulation.

If a person that believes in all of these things but is not Pro-life. Is this person a RINO? How about people that  believes all these things but believes in global warming?,  How about the ones that believe in civil unions?, how about the ones that want to reach across the aisle and compromise with those Godless Democrats?

Is the fact that Chuck Hagel traveled to Iraq with Obama and openly critisized the party, does that make him a even bigger RINO?

Basically, my question is, What qualities define a RINO?