A web-server is a system which delivers data such as web-pages to a client system.
A web-server delivers data to clients across a network. The most common data is web-content, such as text or HTML files, plus any associated images, stylesheets (CSS), and scripts. Typically, the client is a web-browser, and the delivery occurs via the Hypertext Transfer Protocol (HTTP).
In addition to delivering static web-page data, the web-server may also process it in some way. For example, it may add sensor readings into the page. The page may also include a form which enables the user to submit data back to the server. When the server receives this data, it can act on it in some way, such as updating an internal database, or even controlling connected equipment.
To use an Arduino as a webserver, some kind of networking hardware is required, such as Ethernet or Wifi. This may be built-in to the board (e.g. the Arduino Ethernet), or added using external hardware (e.g. the Ethernet Shield). More information is available in this tutorial.
This tag should be used for questions about using an Arduino as a web-server.