The description provided in Wikipedia.org looks to reply this nicely as below:
In the web-development area, three-grade is frequently employed to send to web sites, usually electronic-commerce sites, which are constructed utilizing three grades:
1. A front end web-server serving static message, and possibly some cached powerful articles. In web-based program, Front-End is the content yielded by the browser. This content might be fixed or created dynamically.
2. A mid dynamic content processing and generation degree application server, like Java EE, ASP.NET, PHP, ColdFusion system.
3. A back end data-base, including both information models and also the data-base administration system or RDBMS applications that handles and supplies accessibility to the information Web Development.