MongoDB is a NoSQL database, whereas Hadoop is a framework for storing & processing Big Data in a distributed environment.
MongoDB is a document-oriented NoSQL database. MongoDB stores data in flexible JSON like document format. The fields can vary from document to document, and it gives you the flexibility to change the schema at any time. MongoDB is a distributed database, so it provides high availability & horizontal scalability. You can perform real-time aggregations & ad-hoc querying. You can easily map the documents to your applications.
To know more go through this blog:
Hadoop is a collection of software which is used to store & process big data. HDFS (Hadoop Distributed File System) is the storage part of Hadoop. HDFS file system stores data in a distributed environment, so that data can be processed in a parallel manner. YARN (Yet Another Resource Negotiator) is the resource manager in Hadoop. YARN is the one which allocates resources to the various job which are getting submitted to Hadoop.
I would recommend you to go through these Hadoop Tutorial & Hadoop ecosystem blog:
I hope this answer helps you!
You can also check out this tutorial video which will teach you Hadoop from scratch: