For Group Owners

If you are a Group owner, you'll see a few new options while posting a new topic.

Make an announcement is similar to a discussion topic but will have a different icon to separate it from the discussions. 

Clicking on the Display at the top checkbox will create the topic at the top of the topics list. Clicking on the Lock checkbox will block the replies option. This is useful if you want to make an announcement and not open it up for comments from the other members.