warning: minor minor spoilers for Harry Potter book 3 after first paragraph; major spoilers after second paragraph!
In J.K. Rowling's Harry Potter series, Professor Remus Lupin is the Defense against the Dark Arts teacher in Harry's third year (book 3). After the disappointments of Professors Quirrel and Lockhart in the previous two years, Professor Lupin is a welcome relief. For one thing, he knows what he's doing and can give the classes real hands-on experience with Defense against the Dark Arts. For another, he's just a nice guy, and Harry immediately finds him to be a good, trustworthy teacher.
One of Professor Lupin's lessons is defense against boggarts, shape-changing entities that take on the shape of whatever they think will most scare the person they see. Using a real boggart, Professor Lupin teaches his class to think of something funny when they see the boggart, which will make the creature change its shape so that it will no longer be scary. This defense works well, and when Lupin has each student confront the boggart separately, some of them manage to defeat it on their first try.
However, when it is Harry's turn, the boggart turns into a Dementor, an almost lifeless thing that feeds off the misery of others by enhancing their most wretched memories. Confronted with the image of such a creature, Harry is unable to defend himself and Professor Lupin must come to his aid. Lupin thinks that Harry is so affected by Dementors because he has extremely painful memories, those of his parents' deaths.
Professor Lupin decides to help Harry defend himself against real Dementors in case he should encounter one again (he first saw one when on the Hogwarts Express) and works with him after hours. Lupin decides to use a boggart again; he tells Harry that when it transforms into the shape of a Dementor upon seeing him, Harry's goal is to produce a Patronus with his wand, which will protect him from the Dementor. (for more details, see Patronus, Harry Potter and the Prisoner of Azkaban, and Dementor)
There is one more very important detail about Professor Remus Lupin: he's a werewolf. Yes, that's right, every time there is a full moon, Lupin transforms into a ravenous, violent wolf. To protect other students, when Lupin was a student at Hogwarts, Headmaster Albus Dumbledore created a place where he could go when he was about to transform where he wouldn't endanger anyone else: the Shrieking Shack.
In Harry Potter and the Prisoner of Azkaban, there is an encounter here which leads to several important developments in the Harry Potter story (for more details, see Sirius Black and Peter Pettigrew). In the end, Professor Lupin must leave Hogwarts because Dumbledore knows that parents will object strongly to their students being taught by a werewolf.