What is the difference between HAVING and WHERE in SQL?

What is the difference between HAVING and WHERE in an SQL SELECT statement?

EDIT: I have marked Steven’s answer as the correct one as it contained the key bit of information on the link:

When GROUP BY is not used, HAVING behaves like a WHERE clause

The situation I had seen the WHERE in did not have GROUP BY and is where my confusion started. Of course, until you know this you can’t specify it in the question.