Create a custom type which describes the structure of the rows returned:

CREATE TYPE func_return_type AS (column_name column_type, ...);

Then create the function:

CREATE OR REPLACE FUNCTION some_func (param_type, ...)
RETURNS SETOF func_return_type AS '
 SELECT columns FROM table WHERE condition;
' LANGUAGE 'SQL';

You can use RETURNS SETOF record in your plpgsql functions to return a set of rows. Here is an example of using pgsql to build a query using a table name parameter:

CREATE OR REPLACE FUNCTION test(text) RETURNS SETOF record AS '
DECLARE
  _table ALIAS FOR $1;
  _mycursor refcursor;
  _row record;
BEGIN
  OPEN _mycursor FOR EXECUTE ''SELECT * FROM '' || _table;
  FETCH _mycursor INTO _row;
  WHILE FOUND LOOP
    RETURN NEXT _row;
    FETCH _mycursor INTO _row;
  END LOOP;
  RETURN;
END
' LANGUAGE plpgsql;

Here is an example of calling such a function (the names and types of the result columns are required when returning SETOF record):

mydb=> SELECT * FROM test('mytable') AS (col1 integer, col2 text, col3 date);