the use of $$string$$ - Mailing list pgsql-sql

Hi,
I just discovered that I can use $$string$$ to account for the problem of 
single quotes in the string (or other strange char's).  However, I noticed 
that the table field contained E'string'.  I actually tried to find info on 
this but I did not find anything.  

Could someone explain what it means or better provide a web link for me to 
discover the info.

Thanks in advance,
Johnf