What is the perfect?
The important thing to remember about English tenses is that they often have more to do with what a person thinks of something than they have about when it happened. (This is why, for example, we can use present tenses to describe the past, the present or the future.) This fact is particularly true of the present perfect. But before we look at the 'present' bit of the present perfect, let's examine the 'perfect' bit. Perfect tenses describe not one time, but two. We use the perfect to show how these two times are connected. For an example of how we use the perfect, let us look at a past perfect

For example:
He was happy last week, because I had remembered his birthday.
In this sentence, we have a past 'he was happy' and a past perfect 'I had remembered'. The use of the perfect in the sentence connects two different times in the past. I remembered his birthday, and he was happy. Which one happened first? Well, I remembered his birthday, and this made him happy, so the remembering came first, and him being happy came second. We use the past perfect to explain which happened first, and to connect the two events. Notice the time indicator, 'last week', which tells us that I had remembered before then.

The present perfect is even easier, if you only remember that it is a present tense. Therefore the present perfect always connects an event in the past with the present. Therefore we do not usually add a time indicator to a present perfect, because the time indicator is always 'now'.

For example, compare:
'I have forgotten my wallet'
'I forgot my wallet yesterday'

Notice that the first example does not have a time indicator, so we know that the time indicator is 'now'. The speaker does not have his wallet with him now. The second example tells us what happened yesterday, but it gives us no information about what the situation is now. So the important thing to remember about the present perfect is that it might have happened in the past, but it is true or relevant now.

The grammar of the perfect
The perfect in English is always auxiliary 'have' and the past participle. As always with the English verb system, the main part of the verb gives the meaning, and the auxiliary the time and (sometimes) the person.
For example:
'I had gone' is a past perfect tense, and refers to fact that I went somewhere before another event took place.
'He has gone' is in the present perfect tense, and says that he (we know it is third person from 'has') went somewhere and is still away now

The present perfect and the past
Something which confuses many people is that the present perfect speaks of the past, but it is referring to now. The reason for this is that the event started in the past, but it, or the consequences are still happening. When we talk about something in the past tense this often means that it has stopped happening, since otherwise we would use the present perfect. A good example of how the use of the present perfect contrasts with the past can be seen in the example below.

Think of a pet dog (Let's call him 'Spot')
It is much better to be able to say
'My dog, Spot, has lived for five years.'
than
' My dog, Spot, lived for five years.'
because in the second sentence, poor Spot has stopped living.

The present perfect is not only about time - it also describes a situation.
For example:
Doctor: What is it, Mrs Smith?
Mrs Smith: My son has broken his finger.
Doctor:Bring him to me at once
Because Mrs Smith has used a present perfect, the doctor knows that the finger is broken now, and something needs to be done immediately. Compare that with the following conversation
Doctor: What is it, Mrs Smith?
Mrs Smith: My son broke his finger.
Doctor: When did this happen?
Mrs Smith: About five minutes ago
Doctor:Bring him to me at once
To get the same information, the doctor has to ask another question, since (for example) young Smith might have broken his finger six months ago, and is now complaining that it did not heal properly.

It's now that is important
When you are in a restaurant, and the person who is expected to pay announces 'I have lost my wallet', the interesting point is not when he lost his wallet, but how you are going to pay for the meal. So we use the present perfect to explain that something in the past matters now.
For example:
Joe: Use the hammer
Jane: Where have you put it?
Jane is not concerned when Joe put the hammer wherever it is, she is only interested in where the hammer is now, so she uses a present perfect.

For example:
Interviewer: Do you have any qualifications?
Interviewee: Yes, I have passed all the exams.
Here the interviewer wants to know if the interviewee is qualified. The interviewer is not interested when the interviewee obtained the qualification - the important thing is that he has it now.

It's now that is true
If you say to me 'I have cleaned my shoes' this not only tells me that your shoes were cleaned in the past, but that they are still clean. If we then walk across a muddy park, you might say 'Oh no! I cleaned my shoes this morning.' This tells me that you cleaned your shoes, but after the walk through the park the shoes are not clean any more.
For example:
Joe: Can I talk to Fred?
Jane: Sorry, he's gone out
Fred not only left the office in the past, but he is still out.

Present perfect and time indicators
As a general rule, you can only use the present perfect with a present time indicator. So 'today', 'now', 'for the past five years' can all be used with present perfect, but indicators such as 'yesterday', 'five years ago', 'in 1969' talk about the past and should be used with past tenses.
For example:
I have been to Paris recently. My knowledge of Paris, or where I have been on my travels is important now
I went to Paris last year. I am going to tell you a story about my trip to Paris
Notice how the tense changes here: I always admired him until now. I have stopped admiring him, so I use the past tense.
I have admired him since then. I started admiring him in the past, and I still do so now.

Present perfect continuous
Describes something which happened in the recent past, and although it has finished, the effects are still obvious.
For example:
It has been raining, so the road is wet.
You have been working, so you are tired
For example:
Joe: Why are you out of breath?
Jane: I've been running

Have you understood? Now that you have been reading all about the present perfect, it's time for some exercises!