The difference between sending emails (with messages and pictures) and posting them on Facebook is the difference between knocking on the door of every house on your book versus putting a great big sign on your lawn.
On the web, that "great big sign" is Facebook. Instead of emailing a message to your brother in Tallahassee, and your high school friend in Paris, etc., you just post the message to what Facebook calls "your wall". Think of it like pinning up a bunch of Polaroid photos with messages written underneath it on a sign that people can walk by. That's how it works.
What people don't like about posting messages and pictures onto the web is that anyone can see them. But this is what makes Facebook so cool - it requires permission can see them. And only you can give that permission, to your friends.
If you have a lot of friends, or a large family, this makes the whole process of sharing information so much easier, and that's why Facebook is so popular.