A Cosmos DB database contains a set of collections. A collection is a set of documents and is similar to a table in a relational database. Cosmos DB stores records as documents that are similar to rows in a relational database. A document contains fields that are similar to columns in a relational database. A document can have a dynamic schema. A document in a collection does not need to have the same set of fields or structure as another document in the same collection. A document can also contain nested documents.