Functional and Non Functional Requirements

Basically functional requirement is something specific the system should do. Functional requirement of a system could be,

• Business rules
• Administrative functions
• Authentication and Authorization
• Reporting

On the other hand non functional requirement describes how system works or how the system should behave. These can be,

• Performance
• Scalability
• Capacity
• Availability
• Maintainability
• Security 
• Usability etc. 

Non functional requirements can be identified as quality attributes of a system. Non functional requirements are defined in terms of metrics (something that can be measured about the system).

Resources
http://reqtest.com/requirements-blog/functional-vs-non-functional-requirements/#conversion-0
http://stackoverflow.com/questions/16475979/what-is-functional-and-non-functional-requirement

image credit : http://www.steffen-zschaler.de/bibliographies/nfp/sommerville.png