Could someone provide a rundown of Sitecore 9 XP SQL Server databases and their purpose? For example "Xdb.Processing.Pools"?
The xDB Processing Pools database performs the following functions:
Stores work items with IDs for newly created contacts and interactions. Work items are added by the xConnect Collection service role and consumed by the xDB Processing role during live aggregation processing.
Acts as a retry mechanism for live aggregation, history aggregation and distributed processing by storing work items with IDs for contacts and interactions that could not be processed and should be retried. Work items are added by and consumed by xDB Processing role.
The xDB Processing Tasks database stores processing tasks related to history aggregation and distributed processing.
The xDB Reference Data database contains marketing reference content for all xDB data, such as definitions and taxonomies.
The xDB Reporting database (Reporting DB) contains data that has been aggregated or reduced by xDB Processing.
The xDB Collection database contains all contacts and interactions, including facets and events.
Marketing Automation database contains marketing automation plan and activity enrollments, and the automation pool.
EXM database stores the email dispatch queue used by the EXM Dispatch roles to handle email generation and dispatching.
You can find more information here: https://doc.sitecore.net/developers/architecture-and-roles/roles/xp/storage-roles/xdb-collection-database.html