Linux is a popular Unix like server operating system distribution, its getting popular as its open source and freely available under GPL license. Corporates looking down to cut costs on software, generally migrate to Linux based operating systems. Linux is available as a distribution, there are many popular distributions which includes RedHat, CentOS, SUSE, Debian etc.
Linux based web hosting is cheaper as compared to Microsoft Windows, altough you may not get support for Microsoft based technologies and database system, there are several alternatives, which includes PHP, PERL, Ruby and Python.
Under Linux, you can use open source database system such as MySQL, PostGreSQL or ODBC. Some of the popular database systems like Oracle, DB2 are also available for Linux platforms.
Under Linux, there are several email service utilities available freely, a popular email server under linux is DevCote, its freely available, if you are using RedHat Linux Enterprise edition, the distribution also includes some premium email services.