Here are some conversations to illustrate the difference:
A: I went to the Ritz Hotel last night.
B: Really, did you have dinner there?
A: No, I just had some tea.
This is talking about a past time.
A: Have you had dinner?
B: No, I haven't. I'm quite hungry, actually.
This is talking about the present time - you're hungry because you haven't had dinner.
I saw this movie two years ago.
This is talking about past time.
I've seen this movie, so I don't want to see it again. I'd rather see another one.
This is talking about the present time - you don't want to see this movie at the moment because you've already seen it.