Document databases are designed to store, retrieve, and manage document-oriented information, typically in a JSON-like format. They are known for their flexibility in terms of data models, which allows for easy storage of complex data structures, and they scale out efficiently, making them ideal for web applications, content management systems, and more. Key/value databases, on the other hand, store data as a collection of key-value pairs and are typically used for simpler data storage scenarios where each key is associated with one specific value. Fields/columns is a term used in relational databases representing the individual data items of a table and does not represent a database type. Tables is also a term associated with relational databases and not a non-relational database structure. The primary key is a specific column within a table in a relational database used to identify each unique record, not a type of database.