A journal can be a book, notepad, or even a program that you can use daily to keep track of your thoughts, events that have transpired, inspirations, rants, and raves. A journal and its uses are pretty open-ended as to what you may do with it. To put it simply, a journal is something in which to record parts of life you want to remember.
Journals can be as diverse as the people that use them. You can pretty much have a journal for anything and everything. They can be utilitarian or as elaborate as a junk journal; it is up to you and your personal preferences.