A Book of Shadows is a book where you can record your own personal magickal journey. You can write down correspondence charts, Moon phases, Sabbats, Esbats, Dreams, candle magick etc.
a good place to start is with the Wheel of the Year.
A lot of people start with a three ring binder, then they graduate to a lovely hand written book of their workings.
I guess you could call it a witch's diary.