There are many differences between messages, posts, and announcements. The differences are listed below.

Messages:

• One on one group chats.
• More informal back and forth conversations.

Announcements:

• Posts which stick to the top of the feed for the time specified.
• Ex: Due dates, announcements about some activities.  

Posts:

• Generic updates posted to the class typically summarizing an activity or lessons learned.
• More formal updates.