Yes, there is a difference.
The simple past tense is used to describe an event that occurred in the past: “Julie got home. Then the cat drank the milk.” Here, the order of events is: (i) Julie got home. (ii) The cat drank the milk.
On the other hand, the past perfect (or pluperfect) refers to an action that occurred before another action: “Julie got home. The cat had drunk the milk.” This time, the order is: (i) The cat drank the milk. (ii) Julie got home.
I hope this helps.