What is Force.com

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 software.

Force.com gives you a platform to create rich, collaborative custom apps fast—without buying hardware or installing software.

Technologies Behind a Force.com Platform App

Multitenant architecture
Metadata-driven development model
API Access
Apex
Visualforce
AppExchange directory
Salesforce Mobile
Sites

Multitenant architecture:An application model in which all users and apps share a single,common infrastructure and code base.
Metadata-driven development model:An app development model that allows apps to be defined as declarative “blueprints,” with no code required. Data models, objects,forms, workflows, and more are defined by metadata.
API Access:APIs provide direct access to all data stored in Force.com from virtually any programming language and platform

Apex:Apex is a strongly-typed, object-oriented programming language that allows developers to execute flow and transaction control statements on the Force.com platform server in conjunction with calls to the Force.com API

Visualforce:A framework for creating feature-rich user interfaces for apps in the cloud.

Salesforce Mobile:An application for mobile devices that provides access to your Force.com data.

Sites:Public websites and applications that are directly integrated with your Salesforce organization—without requiring users to log in with a username and password.

AppExchange directory:A Web directory where hundreds of Force.com apps are available to Salesforce customers to review, demo, comment upon, and/or install. Developers can submit their apps for listing on the AppExchange directory if they want to share them with the community.