Websites have many functions and can be used in various fashions; a website can be a personal website, a commercial website, a government website or a non-profit organization website. Websites can be the work of an individual, a business or other organization, and are typically dedicated to a particular topic or purpose. Any website can contain a hyperlink to any other website, so the distinction between individual sites, as perceived by the user, can be blurred. Websites are written in, or converted to, HTML (Hyper Text Markup Language) and are accessed using a software interface classified as a user agent. Web pages can be viewed or otherwise accessed from a range of computer-based and Internet-enabled devices of various sizes, including desktop computers, laptops, PDAs and cell phones. A website is hosted on a computer system known as a web server, also called an HTTP (Hyper Text Transfer Protocol) server. These terms can also refer to the software that runs on these systems which retrieves and delivers the web pages in response to requests from the website’s users. Apache is the most commonly used web server software (according to Netcraft statistics) and Microsoft’s IIS is also commonly used. Some alternatives, such as Nginx, Lighttpd, Hiawatha or Cherokee, are fully functional and lightweight.