@dpcunningham There are only 2 options at the moment: “Bookshelf” and “mongoose”.
Bookshelf is the JS framework used to connect to relational database systems (in the case of strapi that would be: sqlite, postgres, MySQL (and most likely MariaDB)). You can read more about it here:
https://bookshelfjs.org/
Mongo, on the other hand, is a nosql database and needs a different approach, for which there is mongoose: https://mongoosejs.com/
Depending on which DB you decide to use, you will work with either bookshelf or mongoose, when extending yours models, services etc.
In the strapi docu there are places where sample code for both of those frameworks are side-by-side (more or less, they are “tapped”), e.g. here:
Hope that helps!