What exactly IS earnest money ?

[Steven Mitchell]

So we recently tried selling a house.  The day before closing, the buyers backed out.  No reason was stated.

We only got a quarter of their earnest money.

I was under the impression the earnest money exists to protect the sellers in these situations.  IE sorry the deal didnt go through, heres a little something to help defray your additional holding costs. 

What are the exact rules here? I obviously dont understand what earnest money is.