Development and deployment on Force.com typically takes place in an environment or org, which provides a number of features and services for applications within that environment. There is one type of environments in particular that deserve to be called out:
An environment lets you instantly start developing, testing and deploying your software-as-a-service application, and you don't need to touch a single server, they're provisioned in the cloud. The Developer Edition comes with a number of applications pre-installed, including Salesforce CRM applications like Sales, Call Center, Marketing, and Ideas - in case you want to build extensions to these.
There are many more types of environments (obviously not all of them are free!) - for example there are environments for testing and for unlimited production deployment. These are described in An Introduction to Environments.
When you're ready to begin developing, you'll find that Developer Edition is equipped with the latest developer technologies such as the Apex programming language, Visualforce for building custom user interfaces and controllers, the Integration APIs (REST and SOAP APIs), and more.
Sign up for a Developer Edition by joining Developer Force. You'll get your new environment provisioned immediately, letting you begin coding right away.
When you sign up for a new DE environment, you will now have access to a slew of features. You have direct access to Apex, Visualforce, the Force.com Integration APIs, Force.com Sites, the Database Services, Packaging, Chatter and much more.
A number of licenses are provided for other salesforce.com products as well:
You also get access to Developer Preview technology, as it becomes available.
Limits for Developer Edition:
Developer Edition organizations continue to be available as long as these organizations have some type of activity within the previous six months.
From time to time, special preview technology is made available to the Developer Force community members. These previews are made available only to Developer Edition.
Currently, there are no developer previews.
The test methods for the Apex Class XMLDom available on code.google.com, you can add these to your org if you are planning on deploying code from your new Developer Edition. The required test methods are found at XMLDomTest.