The Present Perfect Tense is used to indicate a link between the present and the past. The time of the action is before now but not specified, and we are often more interested in the result than in the action itself.

THE PRESENT PERFECT IS USED TO DESCRIBE:

An action or situation that started in the past and continues in the present.

An action performed during a period that has not yet finished.

A repeated action in an unspecified period between the past and now. 

An action that was completed in the very recent past, expressed by 'just'.

An action when the time is not important. 

They have met each other this year.

Well, kiddo, I haven't finished my sorry yet.

I have visited exhibitions twice this month. Have you ever visited any exhibitions?

Mom, have you taken the pills?

Thank you! Yes, I have taken them.

Have you ever thought about this idea?

The book she has read before was less interesting.

You haven't called your book!

Really? I have called! Haven't you heard it?

They haven't cleaned the room yet.

She has wiped this window twice this week.