Smarty is a PHP template engine used to facilitate the separation of application code from presentation. Typically, the application code contains the business logic of your application, written and maintained in PHP code. This code is maintained by programmers. The presentation is the way your content is presented to the end user, which is written and maintained in template files. The templates are maintained by template designers, or clients.
Smarty is a web template system written in PHP. Smarty separates PHP, as a business logic, from HTML, a presentation logic, and generates web content by the placement of special Smarty tags within a document (i.e. variable substitution).