What is the difference between 'WHERE' and 'HAVING' clause in sql?

Example:
SELECT state, COUNT(*)
FROM Test
WHERE state IN ('CA', 'LA')
GROUP BY state
ORDER BY state

SELECT state, COUNT(*)
FROM Test
GROUP BY state
HAVING state IN ('CA', 'LA')
ORDER BY state

The WHERE and HAVING produce the same result set. What's the difference? The first query uses the WHERE clause to restrict the number of rows that the computer has to sum up. But the second query sums up all the rows in the table, then uses HAVING to discard the sums it calculated for all states except Texas and Georgia. The first query is obviously the better one, because there is no need to make the computer calculate sums and then throw them away.
Here's the rule. If a condition refers to an aggregate function, put that condition in the HAVING clause. Otherwise, use the WHERE clause.
Here's another rule: You can't use HAVING unless you also use GROUP BY.
SELECT state,SUM(baldue)
FROM Test
GROUP by state
HAVING SUM(baldue) > 0
ORDER BY state