What is the Web Tier?
The Web tier displays product information and personalized content, and makes site content available to customers through page navigation. You can consider the servers in this tier to be in the run-time environment, because they respond to client requests for static (or stateless) Web site content, and run business logic, such as pipeline components, activated by the Web pages.
The servers in the Web tier retrieve data from the servers in the data tier for presentation to clients. Clients access the servers in the Web tier directly or through a firewall, and only access the servers in the data tier through the servers in the Web tier.
Because the most popular activity on any Internet site is browsing, the Web tier handles most of the client requests.