http://en.wikipedia.org/wiki/Comparison_of_file_systems
In a simple sense a journal is much like the word we use in writing. A daily journal would be a collection of notes of one's day. A file system journal is a collection of notes as to what operations were performed or in process. A journal could help the OS to recover what the state of files were in case some issue like power loss happened.
Copy on write is a sort of clone or full image of the file instead of a simple note as to what was done. This clone feature is sort of like a time machine or could be used as such to recover the changes made in some file. A text file that has been edited could have only the changed part saved in some area. If needed the file could be recovered to the original state by the saved image (parts of image even)
This is all very basic.