[GENERAL] Can somebody explain what is the meaning for HashAggregate?

[lin]

Can somebody explain what does the postgres done for the explain of sql shows 
HashAggregate( what is the meaning for  HashAggregate  )?
for example: 


postgres=# explain verbose select oid,relname  from pg_class group by 
oid,relname;
                                 QUERY PLAN                                  
-----------------------------------------------------------------------------
 HashAggregate  (cost=12.42..15.38 rows=295 width=68)
   Output: oid, relname
   Group Key: pg_class.oid, pg_class.relname
   ->  Seq Scan on pg_catalog.pg_class  (cost=0.00..10.95 rows=295 width=68)
         Output: oid, relname
(5 rows)


::  first, seq scan pg_class and return (oid,relname);
   second,  make group (oid,relname),  is it fisrt sort by oid then in the oid 
group sort by relname?
Can somebody explain what does the database done for hashAggregate?


 Thanks,
  wanglin