• Database — A database is the same idea as Oracle Database, except instead of having a group of tables. A Mongo Database is a container of collections
• Collection — This is equivalent to a table in Oracle.
• Document — This can be roughly viewed as a row in standard databases.
• Field — A field in a document, is like a column in a row of data in Oracle.

The key difference to learn is that a standard relational database will define a column at table-level where as a document-oriented database will define a field at document level. Basically each document in a collection can have its own unique set of fields.