A relational database is a structured collection of data that is strictly organized into tables. The dominant language for querying databases is SQL. NoSQL databases, such as MongoDB, do not use SQL, but various other query languages and do not have a strict structure in their tables, which are called collections in the NoSQL terminology.
You can categorize NoSQL databases according to their data model as Document, Key-Value, Graph, and Column-family. MongoDB is the most popular document-oriented NoSQL database that is appropriate for use in web applications.
Document databases were not made for dealing with Microsoft Word documents, but for storing semistructured data.