The activity tab shows all processes on Radarr's "to-do list", "have done list", as well as the blocklisted releases that have failed.
The queue shows all items the application can recognize that is in the specified download client's category (Settings => Download Client => Category). To view all releases Options => Show Unknown. This is not stored anywhere within the application, but is updated via your Download Client's API responses.
More information about the Queue section Coming Soon - Contributions are Welcome.
The history tab shows all previous operations that Radarr has completed (regardless of successison). This includes any operation that Radarr can do via user input, such as failures, grabs, imports, upgrades, and human input requiring completed downloads (seen as a yellow import icon).
Blocklist is formerly known as 'Blacklist'
The blocklist tab shows all blocklisted releases that have failed and have been automatically blocklisted, or via user input in the queue tab. You are able to remove individual blocklists or clear the entire list.