The details are here.

LibraryThing.com is a social networking and book cataloging site for people who love books.

A free account limits you to cataloging (and reviewing etc) 250 books (or CDs or DVDs).

If you download either the Android app or the iOS app, you can have a free lifetime (and unlimited) account, which usually costs 25.00.

If you already have a LibraryThing account, using the Android or iOS app to log in gives you a free lifetime account.