has and have
has and have, when used alone, are both used to describe a recurrent state of possession, or participation:
When deciding whether to use has or have:
has is used only with a third- person subject: that is, you, he, she, it, or any named subject (John, Australia, the Queen)
have is used only with a first- or second- person subject: that is, I or you.
have had and has had are used, as noted by Veronica Diamond in her answer (which has now been deleted), to describe something that has happened in the past.
The technical term for this is the present perfect. In perfect is a technical term meaning completed, and so it roughly means: something that has been completed - now.
As mentioned by StoneyB, in this form, have is a auxiliary ("helping") verb, which temporally "sets" the time for the main verb, which can be any verb, not just "have".
This may take the form of a circumstance of time that links it to the present:
- They have had three exams in the last week
- She has had three exams in the last week
However, this circumstance is not obligatory:
- I have had dinner at that restaurant a dozen times.
- She has had dinner at that restaurant a dozen times.
In the examples above, I (or she) could have had dinner at the restaurant a dozen times fifty years ago, or in the last year. But it's not necessary to provide a timespan.