URL Redirection and domain forwarding allow redirection of web traffic from one site or page to another.
URL Redirection
One of the simplest ways to redirect visitors from one URL to another is to set forwarding at the page level, using the meta refresh tag within the head section of the actual HTML page. However, some search engines will refuse to index a page with a high meta refresh rate, and Google (I think) will not index pages with any redirection whatsoever.
Wikipedia provides a fairly comprehensive list of URL redirection techniques.
Domain Forwarding
While URL redirection works at the page level, domain forwarding works at the sever level. This means visitors will not experience any additional loading for the website that is being forwarded to. Furthermore, it is possible to use masking or cloaking to prevent the URL of the site you forwarded to from displaying in the browser’s address bar.
There are several different ways in which forwarding can be implemented on the server.
Examples
Wordpress, for example, offers free hosting of blogs, using URLs such as UserName.WordPress.com. URL redirection could be used to forward the page www.MySite.com/blog.html to UserName.WordPress.com.
Alternatively, domain forwarding could be used to forward the URL blog.MysSite.com to UserName.WordPress.com. If masking/cloaking is set the, this redirect would be transparent to the user.
Dub, Dub, Dub and other subdomains
Universal Resource Locators (URL) are generally of the format http://www.MySite.com. The MySite.com part of this address is called the domain name, while the www prefix is called the subdomain. It is generally possible to have any number of additional subdomains, such as blog.MySite.com, wiki.MySite.com or mobile.MySite.com, under a domain. In fact internet service provides routinely create a number of standard subdomain, such as ftp.MySite.com and mail.MySite.com names under each hosted domain.