In order for people from all over the world to view your webpages you need to have them located on a computer system that has a continuous high speed connection to the Internet. Servers, along with their high speed connections, are very expensive to purchase and maintain. So unless you have an unlimited budget, most individuals, organizations, and companies opt to have their website located on someone else's servers at a fraction of the cost. These websites are considered to be hosted by virtual servers. Websites on a virtual server account have their own IP address.
Websites setup with virtual hosting share their IP address with many other sites.
This has been known to cause problems with certain search engine indexing spiders and some web browsers. Virtual hosting also severely limits your ability to configure your server and can not offer the same type of features that a virtual server offers. They are simply regular email accounts that may be used under your domain. They may also be setup to forward mail to an alternate email address. An autoresponder is an email account that automatically emails a reply to anyone who sends an email message to it.