Systems Notebook: What’s in That Container?

Have you ever opened your refrigerator to get a tasty snack and caught sight of that one container in the back, the one that is unmarked but you know has been there since sometime last June? And as you close the door, you kind of wonder if it just moved a little? Have you ever felt the same way about Linux containers running on your servers? What exactly is in there? And how did they get there in the first place?