Not to be confused with...
... 'email' or 'blog' ... An 'email' is not private, but is usually restricted to the person to whom you are writing. You want to share your personal thoughts and feelings, perhaps about something that has just happened to you, but only through a private conversation limited to a friend.
A 'blog' is essentially a very public sort of journal - you intend to present some of your personal ideas in a lively entertaining way to a general, undefined audience. You want to entertain, and hook your (unknown) audience by your interesting, even provocative, opinions and experiences.