When you follow another user, it gives you the ability to see their "actions" in your Dashboard's Activity Feed. So, you'll be able to see whenever someone posts to a blog, bulletin, property directory, company directory, article directory, OR when they update their status, add a follower or colleague, comment, etc.
Followers do not have the same abilities as colleagues. The main difference is that colleagues have the ability to send private messages to one another. Additionally, if a person has their profile set to be private, followers wouldn't be able to see the information, whereas colleagues would.
Think of colleagues as firm, 2-way relationships, whereas followers are merely a 1-way relationship.