"some_identifier" vs ":some_identifier"?

I'm just not getting the Symbol type. What does this buy me? When would I want 
to use it? What is it analogous to in another language?

I understand how to use the colon with attr_accessor through rote memorization; 
but I don't really understand it.  :-(

Can someone provide a very simple example of its proper use and advantages?

Thanks _very_ much for your patience,
Christopher
 
 
Christopher J. Meisenzahl CPS, CSTE
Senior Software Testing Consultant
Spherion
christopher.j.meisenzahl / citicorp.com