What is the „Plusquamperfekt, how do you use it and how can you tell it apart from the „Perfekt“?

Both the “Plusquamperfekt” and the “Perfekt” are past tense, each of them describing a different level of the past – highlighting the relationship between several actions in the past.
The “Perfekt” is the common past tense used in written language. It is used for actions that happened and are completed now in the present.
For example: “Ich habe getanzt” -> Meaning: I danced at some point in the past. Now I am not dancing anymore.
The “Plusquamperfekt” on the other hand is used to describe actions that happened before other actions in the past. Therefore, sentences with the “Plusquamperfekt” are also in need of another form of past tense, like “Perfekt” or “Präteritum”.
For example: “Ich hatte getanzt, bevor ich nach Hause gekommen bin” -> Meaning: I danced, and after that I came home.
As you can see, the sentence contains to actions: One in “Plusquamperfekt” (Ich hatte getanzt) that happened first, and one in “Perfekt” (Ich bin nach Hause gekommen).
Let’s look at the example, to see how you can construct the “Plusquamperfekt” tense:
It contains only two key elements: A variant of “sein” or “haben” in past tense (in this case: “hatte”, as variant of “haben” in the past tense of “Präteritum”), and the Partizip II of the verb that describes the action (in this case: getanzt). The Syllable “ge-“ at the beginning of the word is usually a sign of the Partizip II.
As you can see, “Perfekt” and “Plusquamperfekt” are much alike, only differing in the tense of sein/haben; a very minor difference, that can have massive impact on the meaning of the sentence.