What is force.com? Or what is salesforce?

The force.com platform is the world’s first Platform as a service (PaaS), enabling developers to create and deliver any kind of business application in the cloud entirely on-demand and without any software.

Why is force.com? Or advantages of force.com?

Force.com allows us to develop both data-centric and collaborative applications

A data-centric application is an application that is based on structured, consistent information such as you might find in a database.

A collaborative application is an application with data and services that are shared by multiple users in different locations. These applications can be accessed from anywhere in the world with only a web browser.