The 3 Domains
Page 23 is a sidebar showing "Biological Classification." The page correctly points out that the most fundamental taxonomic split of organisms is into the three domains: Bacteria, Archaea, Eukaryota. Of these domains, Explore Evolution says, "Each of these has a fundamentally different cell structure."
While true, this is only half the story--the most important differences are genetic, not morphological. In fact, bacteria and archaea have such a close superficial resemblance that for a long time they were not distinguished into different domains.
The 3 Domains model is relatively new; it was proposed by Carl Woese in 1990. (Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya.". Proc Natl Acad Sci USA 87 (12): 4576 9.)