What is the purpose of a journal entry? Is it for when I send out a book or when I find a book?
Hi, and welcome! If you haven't already done so, do check out the "information" tab - the "How to BookCross" section and the "Frequently Asked Questions" page may be helpful.
In short, a journal entry is a way to add your comments on a book, whether you're registering your own copy or commenting on someone else's book that you have found. If you want to release a book, there's a "release note" option - release notes are a special type of journal entry that includes date, time, and location information. You don't have to make release notes, but by doing so you can specify the location and have your release show on the "go hunting" pages, where it might attract the attention of local book-hunters.
Here's an example of a book that has had both journal entries and a controlled release note and wild release notes, which are really just a specialized kind of journal entry. As someone else pointed out in another answer to you, you can do either a wild release note (used when you release the book in the wild and don't know who will pick it up) or a controlled release note (used when you are giving the book to someone in particular, like a friend or another BookCrosser). http://www.bookcrossing.com/---/11093479/
It may sound confusing, but it becomes very easy with practice. Enjoy!