Wikipedia (somewhat redundantly) defines commonplace books as "a way to gather knowledge, usually by writing information into books" and goes on to describe their contents as including "notes, proverbs, adages, aphorisms, maxims, quotes, letters, poems, tables of weights and measures, prayers, legal formulas, and recipes." Critically, "entries are most often organized under subject headings and differ functionally from journals or diaries, which are chronological and introspective." I apologize for the blatant Wikipedia plagiarism; I had to be sure we started on common ground.
This is my commonplace book. Broadly defined by intention, it comprises things I encounter that catch my eye or stick in my mind.