Redirection: When a specific URL redirect your browser to a different URL to show a different page is known as redirection. Redirection isdone for different purpose. When a site is moved to a new URL then redirection is needed. This redirection may be of two types: 301 redirection or permanent redirection and 302 redirection or temporary redirection. In case of 301 redirection a rsponse code is generated which tells visitors that the original page is permanently removed. SEs like this method and reading the available information they pass the PR value and link popularity to the new page or to the new site. So many reasons are there for redirection. Such as redirecting visitors to browser specific versions or when a site is shifted to new domain. Redirection is not bad for search engine as long as they are done for specific valid reason. For example if we try for http://www.aj.com/ we will immediately redirect to ask.com (AskJeeves) or if we try for http://www.av.com/ we will redirect to altavista.com. Also big seach engine or site provides local versins for its site, then sometimes we redirected to the local version of the site.
But if a redirection is used to mislead visitors then this process is known as spammy. Sometimes spammers bulid up one site optimize that site for search engine only but redirect visitors to some other site or provide misleading information by redirecting visitors to some other site.
Mirror Site: When two different URL contain same content that is known as mirroring. Ex: http://ch.dmoz.org/ is a mirror site of http://dmoz.org/. That is when a site replicate an existing site is known as mirror site. Purpose of creating mirror site is load balancing or reducing network traffic or hits on server. Mirror site is highly useful when original site generates huge amount of traffic for a single server. Sometimes big site maintains local data center for load balancing. Any user can download files or can access a site more speedily from their local server.
Xyona.com and threadnet.com are also mirroring each other. Mirroring it self-having another meaning in the world of www. Sometimes duplicate content placed in different domain for different keywords are only created and used to gain search engine ranking. This method is a Sammy method. For more details check this out.
Duplicate Content: Copying content from original site and placing it in another site is known as content duplicity. Or same content placed in many network sites is also considered under content duplicity. Any search engine love to crawl fresh content, and they consider copied content under spamming. Sometimes they take tough decisions against this type of approach.
Cloaking: This is a technique of delivering one page to search engines for indexing and an absolutely different page for visitors. This is also known as stealth. Web master scripted their server for search engine spidering in order to gain search engine results and deliver some other pages to their visitors. However search results are also misleading, as Meta tags do not match up what exactly exists on the page. Few search engines even ban cloaked sites. In case of URL cloaking full size frame is used for displaying targeted page. Reason behind using frame is to hide the URL of the displayed page. Cloaked URL is also known as “vanity URLs”.
Doorway Pages: Bridge pages, portal pages, entry pages all are the synonyms of doorway pages. Doorway pages are specially made for search engines. Webmasters create pages for a particular phrases and optimized it for specific search engines but do not provide much information. Send its visitors to the main site by providing a prominent “Click here” link or with a fast Meta fresh command.