Visual Force is a framework which allows salesforce developers to build  custom user interfaces which can be hosted on the Force.com platform.  The visualforce framework consist of tags similar to HTML.
In visualforce language, each tag corresponds to a fine-grained user interface component like section of a page, or a field or a related list .
The behavior of Visual force components can either be controlled by the same logic which is used in standard Salesforce pages or it can also be controlled by developers who can associate their own logic using a controller class written in Apex.
Some Importants points regarding visualforce:

• We can write HTML, CSS, and Javascript inside a visualforce page.
• A visualforce page consist of tags/components and controllers.
• Every visualforce tag has attributes and attributes define the properties of the tag.
  By defualt, every visualforce tag has two attributes.
  1) ID: this attributes binds the two components together.
  2) Rerendered: This attribute is used to show/hide a component.

 
When and where to use Visualforce?

• Visualforce pages can be used to override standard buttons for e.g.: we can create new buttons for accounts, override save button in contacts.
• We can embed components in detail page layout using Visualforce.
• Visualforce gives the flexibility to add dashboard components on custom help pages.
• You can use Visualforce to build wizards and other multi step process.
• Use Visualforce to create your own custom flow control through an application.