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What is MongoDB?
MongoDB is an open-source database developed by MongoDB, Inc. MongoDB stores data in JSON-like documents that can vary in structure. Related information is stored together for fast query access through the MongoDB query language. MongoDB uses dynamic schemas, meaning that you can create records without first defining the structure, such as the fields or the types of their values. You can change the structure of records (which we call documents) simply by adding new fields or deleting existing ones. This data model give you the ability to represent hierarchical relationships, to store arrays, and other more complex structures easily. Documents in a collection need not have an identical set of fields and denormalization of data is common. MongoDB was also designed with high availability and scalability in mind, and includes out-of-the-box replication and auto-sharding.

Feature Comparison
Like MySQL, MongoDB offers a rich set of features and functionality far beyond those offered in simple key-value stores. MongoDB has a query language, highly-functional secondary indexes (including text search and geospatial), a powerful aggregation framework for data analysis, and more. With MongoDB you can also make use of these features across more diverse data types than with a relational database, and at scale.

Why use MongoDB instead of MySQL?
Organizations of all sizes are adopting MongoDB because it enables them to build applications faster, handle highly diverse data types, and manage applications more efficiently at scale.

Development is simplified as MongoDB documents map naturally to modern, object-oriented programming languages. Using MongoDB removes the complex object-relational mapping (ORM) layer that translates objects in code to relational tables.

MongoDB’s flexible data model also means that your database schema can evolve with business requirements. For example, schema changes that took days of weeks in The Weather Channel's MySQL databases could be made in just hours with MongoDB.
MongoDB can also be scaled within and across multiple distributed data centers, providing new levels of availability and scalability previously unachievable with relational databases like MySQL. As your deployments grow in terms of data volume and throughput, MongoDB scales easily with no downtime, and without changing your application. In contrast, to achieve scale with MySQL often requires significant, custom engineering work. Baidu migrated from MySQL to MongoDB to support its rapidly growing business. The Chinese internet services giant now powers over 100 apps and manages in excess of 1PB of data with its MongoDB cluster.
source : https://www.mongodb.com/compare/mongodb-mysql