The Catalogue is for finding books that are available through the library, both hardcopy and eBook versions. Take a look at How do I find and access eBooks? for instructions.
Discovery will search certain eBook databases (see FAQ below for more information on this) but it will also search online journals and databases so can be used to find journal articles, newspaper articles, book reviews etc.
So while Discovery will search for books in the library it is best to use the Catalogue for this while using Discovery for online resources like journal articles.
