I'm in two now, and I've been in others in the past.

In the two of that I'm part of right now, we take turns hosting in groups of 2-3 at someone's house. The hosts provide food and wine. We spend about 20 minutes talking about the book - sometimes more, sometimes less - and spend the rest of the time socializing.

I suspect a book club in a library will have less emphasis on food and socializing, and more book discussion. But no one will be quizzing you about how much you read or understood. It's generally all very lowkey. People are just there to have a nice time.