With as few spoilers as possible, ghouls look just like humans but they eat them. As one would expect, these ghouls are quite strong and their origins are a mystery - similar to vampires in other stories. The major difference being that people are very aware of ghoulish existence.
As with many stories, this introduces an introverted, whiny weakling who is thrust in the midst of the ghoul world. Many things then start happening and it seems like the author or (if the anime strays from the manga) producer loses track of things and doesn't tie up loose ends.
