Best to keep these questions apart, although related.
In short : Pages is like creating standalone content; informative or describing aka like a story (fill with your likings or meaning)
As being a cms the core = content > Cms
In v1 the content plugin was in use, but now rewritten.
Books and chapters allow you to 'gather/combine' pages into a 'common'markup (read books? They have ttiles, chapters, pages) look at them in the same way when using the system..So you can have 100 pages where 20 belong to book A 15 to B etc.. You can (url) call books and related pages etc...and so on (versatile).
Menus : build there to add own 'small content goal) to assign smaller pieces into (normally) the sidebars of the system, can be a clock, small image, little poll etc.. Some are provided in core, others you can add. The menus build can than be set to an area of your liking.. Multiple languages add some versatile possibilities too when a 'menu' is in other languages also of importance...
It is hort but that is the basics... Need more info : just ask