Blogging is completely unrelated to the game itself. There are no moodlets or rewards in the game for doing it. Players write blogs just for the fun of it and to share their stories. The Sims community seems to enjoy reading about others' adventures and like sharing stories and pictures. You can set up a blog on the official Sims 3 site, or you can use a free blogging tool, such as WordPress or Blogger. I have a blog and I admit that it takes a lot of effort to maintain it, which makes it not up to date most of the time.
If you have questions about things, don't worry about posting them. It's not a dumb topic.