The difference is that WHERE operates on individual rows, while HAVING operates on groups.
You can have WHERE without HAVING, you can have HAVING without WHERE, you can have both WHERE and HAVING, and you can have neither WHERE nor HAVING. But you can't have HAVING without grouping, even if the group consists of the entire result set.
A good example of the difference between WHERE and HAVING is given in my earlier answer Why WHERE cannot be used after GROUP BY.
Typically you will have at least one column to GROUP BY, such as:
select cust_id , count(distinct order_id) as orders from sales where order_date >= '2005-01-01' group by cust_id having sum(order_amt) > 10000
This query returns the cust_id and number of orders for every customer who has order totals of $10,000 or more (the HAVING condition) for orders made this year (the WHERE condition).
An example of the entire result set as a single group is:
select count(*) as orders from sales where order_date >= '2005-01-01'
This query returns the number of orders this year. The aggregate function COUNT(*) operates on the single group consisting of all the rows that satisfy the WHERE clause. Note that it would be unusual to have a HAVING clause in this situation.
This was first published in February 2005