Learn MongoDB from scratch Absolute beginner’s introduction

MongoDB is a document-oriented DBMS, with JSON-like objects comprising a data model, rather than RDBMS tables. MongoDB does not support joins nor transactions. However, it features secondary indexes, an expressive query language,
atomic writes on a per-document level, and fully-consistent reads. MongoDB uses BSON, a binary object format similar to that
but more expressive than JSON.
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MongoDB uses dynamic schemas. We can create collections without defining the structure, ie the fields or the types of their values, of the documents. You can change the structure of documents simply by adding new fields or deleting existing ones. Documents in a collection need a unique set of fields.

MongoDB database stores its data in collections not in tables. The collections are the rough equivalent of RDBMS tables.
A collection holds one or more documents, which corresponds to a record or a row in a relational database table, and each document has one or more fields, which corresponds to a column in a relational database table.

As one of the NoSQL database engines, MongoDB is a fairly trendy product at the moment. MongoDB provides for a fairly permissive database experience (compared to a relational database). The MongoDB schema is dynamic, which allows you to insert data in a very flexible way.
One of the key uses for MongoDB is storage of unstructured data, such as from sensor arrays.
MongoDB can comfortably chomp away at large data streams, providing a resilient repository.