Mongodb:

Structure

1. mongod 
	- daemon (background process program) that performs all server functions (e.g. taking in client requests, serving responses, etc.)

2. database
	- group of collections
	- mongod can have multiple databases

3. collection
	- group of documents

4. document (json)
	- group of fields

5. field
	- key-value pair
	- value can be an array

6. array
	- group of items


Other

index
	- forces existence (if it's not a sparse index) and uniqueness of values (null if doesn't exist) for a particular key in a collection
	- allows unambiguous query of said keys
	- mongo automatically creates an index on the _id key (similar to row_id in sqlite), but more indexes can be created