Framework is one of the most important abstraction of Mesos that differs Mesos from other resource scheduler. A traditional resource scheduler has full control of which resource to offer for which application running on it and it is the scheduler that decides where a job should be run with how much resource. Mesos however is a 2-level scheduler. Not only the master scheduler can make decision, the application itself can also accept or reject offers from the master scheduler. This has many advantages:

• Simplicity: instead of having a one-size-fit all monolithic scheduler, the Mesos scheduler can be kept down to minimum, pushing specific scheduling logic to the framework itself.
• Scalability: since the master scheduler doesn't need to do everything, it can handle more load, scaling out to thousands of nodes
• Extensibility: you can write custom framework that implements its own scheduling mechanism or make use of Mesos rich APIs to do custom logging, monitoring, etc

Let's look at a Mesos Framework in details: