You can think of guides like Matryoshka dolls.
Guides are the largest, outermost Matryoshka doll - they contain all of the other dolls, or in this case, pages, boxes, and content.
Pages (or Tabs) are the 2nd Matryoshka doll. They provide structure to your guide and the content within it.
Boxes are the 3rd Matryoshka doll. They contain the actual content you want to share. There are 4 different types of boxes in LibGuides:
Content items are the smallest of the dolls. This is what you put inside your boxes. Content items include text, links, databases, books, videos, widgets, RSS feeds, polls, etc.
A guide can contain many pages, a page can contain many boxes, and a box can contain many pieces of content.