Explain what is REST and RESTFUL?

REST represents REpresentational  State Transfer; it is a relatively new aspect of writing web API.

RESTFUL is referred for web services written by applying REST architectural concept are called RESTful services,
it focuses on system resources and how state of resource should be transported over HTTP protocol to different
 clients written in different language.  In RESTFUL web service HTTP methods like GET, POST,
  PUT and DELETE can be used to perform CRUD operations.

2) Explain the architectural style for creating web API?

The architectural style for creating web api are

HTTP for client server communication(HTTP is a set of protocols designed to enable communication between clients and servers.
 It works as a request-response protocol between a client and server)
XML/JSON as formatting language
Simple URI as the address for the services
Stateless communication

Mention some key characteristics of REST?

Some key characteristics of REST includes

REST is stateless, therefore the SERVER has no state (or session data)
With a well-applied REST API, the server could be restarted between two calls as every data is passed to the server
Web service mostly uses POST method to make operations, whereas REST uses GET to access resources

What is the purpose and format of URI in REST architecture?
Ans: Purpose of URI is to locate resources on the server that are hosting web services.
Format of URI: <protocol>://<service-name>/<ResourceType>/<ResourceID>

Advantage of statelessness:
Each and every method requests are treated independently.
Application design is simplified as it does not maintain client’s previous interaction.
Works with HTTP protocol as it shares the feature of being statelessness.

The disadvantage of statelessness includes

Every time client interaction takes place, web services are to be provided with
extra information about each request so that they can interpret the client’s state.