I agree with Julia's answer, but I am going to answer this because I like to practice explaining this concept. The question comes up a lot!
Refers to an event that happened and then stopped happening. Use this when you do NOT need to convey a relationship to status at the present time, nor about this event in relation to any other event.
"I passed the test."
"She left the children here with me."
( know you didn't ask about this, but it is part of this explanation!)
Refers to an event that happened in the past and is still happening at the present moment, or the EFFECTS of this event are still happening at the present moment. The lasting effect may be unspoken, but is implied by the use of present perfect.
"I have passed the test (so I am allowed to attend graduate school)."
"She has left the children here with me (while she is out of town for a few days)."
Used when you have a linking relationship between two events (or states of being), BOTH of which are in the past, even if you do not directly speak of the second event.
"I had passed the test (so I was allowed to attend graduate school last year)."
"She had left the children here with me (while she WAS out of town for a few days)."