MySQL Db is a structure Db for traditional application usage (or old application) such as ERP system, CRM system, etc. MongoDB is a non-structure Db for nowadays big application such as Instagram, etc.

Characteristic comparison as below:

MySQL: More Mature, Table Structure, Requires a Schema, Great with Relationships, Scales Vertically.

MongoDb: Shiny and New, Document Structure, More Flexible to Changes, Not Great with Complex Relationships, Horizontally Scalable.