When should I use a Campaign, and when should I use a Message? What's the difference between the two? I thought messages are campaigns?
You two are on the right track- one main difference here is that campaigns are made up of a series of individual messages.
Both can be used to target to a specific audience and are delivered at a chosen schedule; however, Campaigns also have additional features such as message prioritization and rate limiting.
We have a resource here that dives deeper into the differences between Messages and Campaigns.
Hope this helps!