What is Visualforce
Visualforce is the component-based user interface framework for the Force.com platform. The framework includes a tag-based markup language, similar to HTML. visualforce provides richset of component and also provide way for developers to create their own components. Visualforce uses model-view-controller (MVC) paradigm. and also use auto-generated controllers for database objects, providing simple and tight integration with the database. Developer can write there own controllers, or extensions to controllers, using Apex Code.
How to create Visualforce page
Visualforce page is tag-based markup language page that is use group of markup tag for creating web page.
For creating visualforce page you have to register with salesforce.com and after registration login with username and password like below pic.
After login go to the Build section and expend develop link and click on visualforce page link.(you can findout Build-> develop-> visualforce page
link in left side menu of salesforce page)
Build-> develop-> visualforce page.
Now Click on new Button for creating new visualforce page.
Now enter Label,Name and Description of visualforce page and click on save button(you can write code in Visualforce Markup section).
your new visualforce page is created and you can see your visual page by clicking preview button.
How Visualforce page works and security
this digram is showing how visualforce page work(intract with classes and database) and related security like page lavel security , object lavel security and field lavel security.