The "Currently Online" side bar item is a quick reference to letting you know who/how many people are online. This is similar to the Currently in Chat side bar directly above it. Currently in Chat allows you to see exactly who's in chat right that moment. If you were to log in, you could navigate to the chat area and begin a dialog with them.
On the Home page you'll find "Most Recent Blogs" and "Active forum posts". Most Recent Blogs is a general side bar for any and all blogs on the site. If you were to post a blog, you would see it update in that sidebar the next time you refreshed a page. Active forum topics does much the same in that it shows the 6 most recently commented on forum topics. Clicking anywhere on any of those links would take you directly to the forum topic/blog in question.