Yep, thats exactly what it is...
At book group, for a few weeks we would post ideas on what we should read next, there would be a sort of poll to decided on what that book would be.
We would set a time limit to read the book, and have a bit of a ching wag about it at the end, and/or when reading it.
It could be a book that you have at home already, one that you're going to buy or a library book. genre has to be decided upon, it could be a classic, or something thats in the book charts at the moment, a debut author or an older title that we just never got round to reading or maybe a book from a series, outlander, roy grace, rizz and isles etc etc.
when we did challenges, there would be a specific genre, or a series. for eg..
i did a southern challenge. i had to read books that were set in southern states/ by a southern author. you can choose how many you wish to read and choose your own time scale to read them in. it's a good way to introduce new titles/ authors to other people...