Daniel's journal is a collection of notes kept by Daniel Faraday since his time as a professor of physics at Oxford University's Queen's College Physics Department. During the events of "The Constant", while trying to help Desmond, Daniel is seen frantically looking for the journal, and uses it to aid Desmond when Desmond became unstuck in time. Daniel also used the journal in "There's No Place Like Home, Part 1". Caesar later found a photocopy of one of the pages of the journal in a folder of assorted maps and notes at the Hydra facility. The journal is also the topic of a deleted scene from the Season 4 DVD.
Just warning you, that if you haven't watched the whole show might see some information before you are ready. To help with that we have this nifty filter. Just set it to the episode you you are upto, and we'll do our best not to spoil anything.