What is the difference between web service(i.e. gmail.com) and cloud service? Could be web service treated as particular example of cloud service?

No, they are not the same. However, they are increasingly dependent on each other if users want the full speed and ease of use effect that such services offer. Technically, organizations can have a Web service without a cloud service or vice versa, but one gets greater business efficiency when the services are combined. Web services are the tools that allow users to interact with software over the Internet. Cloud services are the servers that store the data, security and other infrastructure pieces needed to allow Web services to provide unique value as Web-accessible applications. The two services are currently mingling, intertwining and becoming nearly inseparable. Even though they aren't the same thing, they both work together to provide the maximum value of using either for a business. An observer can see the shift as all sizes of software companies offer a combined all-in-one Web and cloud service package. It's like bundling in the insurance business. Bundling gives users the maximum impact of the service, while the parts can still be used separately if desired.Reference: http://searchsoa.techtarget.com/tip/Web-services-vs-cloud-services-Are-they-the-same