I recently used mongoDB for a project and I am quite impressed with its speed and MongoDb is a popular no-sql database .For huge data Google uses its own database structure i.e Big table mongo db has too many pros over MySQL but also cons.It all depends on what your pupose would be.MongoDb is definitely not a RDBMS hence in a case where you would be using too many joins to get your data from different tables you need a sql database . In a project where you have huge number of records and constants writes and reads mongoDb would be a write option for you.
In mongoDB collection is similar to the tables in collection and collection would contain several documents which is a BSON document values are stored corresponding to a key and key may vary from every document in a collection this makes it flexible for varying type of data. If your DB is 3NF and you don’t do any joins (you’re just selecting a bunch of tables and putting all the objects together, AKA what most people do in a web app), MongoDB would probably kick ass for you. But If I need ACIDity, I won’t use NoSQL.