Simple past tense is the tense we use when telling events that happened in the past - usually in the order in which they occurred.
Last night, I ate pizza then saw a movie.
Past perfect is the tense used to tell of events that have happened in the past at some point, but not necessarily a specific time or in a specific order. Also, past perfect is used if something happened in the past up until a certain point, then changed.
I had eaten pizza and seen movies before, but nothing was as good as what I ate and saw last night.
Simple past is one of the two basic tenses in English grammar, and it is very common. Its normal purpose is to refer to an action which was completed before Now: I ate my sandwich. I walked the dog. The Spanish Armada was defeated in 1588. We use it to tell others what we did, we use it to narrate stories, we use it to transcribe the passage of history, and so forth. It is a simple statement of a finished action.
The past perfect verb form, on the other hand, is used to express a past action which predates another past action: When I had eaten my sandwich, I walked the dog. The key is that there must be two past events occurring at different times which are being related, though they needn't be in the same sentence. Simple past need describe only one past event.
Past perfect is a much less frequently used verb form. Though it has a safe refuge in formal writing, it is being replaced by simple past in many less formal situations where other structures can do its job. It is not used so often these days, because language users have found that the order of past events can just as easily be assigned by using prepositions, for instance (After I ate my sandwich, I walked the dog). With 'after', the sequence of events is clear enough: first the eating, and then the walking.
Some grammarians will still insist on using past perfect in sentences like the above, but it too often appears overly formal in contemporary writing.
Today, the 'mandatory' uses of the present perfect are in cases of (1) potential confusion about event order (? When I ate my sandwich, I walked the dog-- is the eating and the walking simultaneous or consecutive?) and (2) a desired emphasis on the precedence of the first event (After I had already eaten my sandwich, Mother told me that she was going to order a pizza!-- here the speaker wishes to emphasize that the sandwich was gone and the stomach was satisfied before the wonderful vision of the pizza arose).
I have included a link to a simple exposition of the past perfect verb form below.