This detailed guide to the U.S. Code is the source for this answer.
Titles are the largest organizational units, with each title focused on a fairly broad, but closely related area of legislation. For example, Title 17 is Copyrights, Title 35 is Patents, Title 18 is Crimes and Criminal Procedure.
Within each chapter, the main unit of organization is the Section (§). For example, 35 USC §101 is the section that outlines subject-matter-eligibility for patents.
Between the Title level and Section level, the U.S. Code uses chapters, subchapters, parts, and subparts, not necessarily in that order, and not consistently from title to title. Title 35 is divided into Parts, then Chapters, then Sections. Title 17 is divided into Chapters, then sometimes Subchapters, then Sections. Title 26 is divided into Subtitles, then Chapters, then Subchapters, then Parts, then Sections.
In the end though, you always get to Sections.
Sections can be subdivided quite finely into subsections, paragraphs, subparagraphs, clauses, subclauses, items, subitems.
Here is an example of a section that is organized quite finely: 26 USC §401. It includes divisions all the way down to subclauses (the things identified with capital roman numerals: I, II, III).