Web tier 

The web tier normally hosts the static pages and assets required by the application. This includes the HTML files, the CSS, the images, and any other static content. The web tier by its nature should be stateless and hence can be horizontally expanded. 

The web tier is implemented by an application, such as Apache, NGNIX, IIS, and so on. We normally see that with high volume websites, we may choose to add a cache (memcached or similar) to further enhance the user experience.