A custom database in Admin → Databases lets you define your own fields and get list and detail pages for records made from them.
The test is whether the content has structure. A staff directory has a name, a role and a photograph; a recipe has ingredients and a time. Those are fields, and a database gives you consistent entry, consistent display and the ability to filter.
A thread where people talk about recipes is a forum. A collection of recipes people look things up in is a database. Records can have comments, so you are not choosing between structure and conversation.
Individual fields can be restricted to groups, which is how you keep a private note on a public record.