This means that your website is now removed from a search engine's index. There are several possible reasons for this, including extended downtime and techniques perceived by search engines to be ‘spam.’

A link to a page that does not exist, probably because the page no longer exists, the page has moved, or the server is down. Automated programs can be used to check your website to ensure such dead links do not exist. A dead link leads to a '404 error page.'

Pretty self-explanatory, it is descriptive text which concisely explains the purpose of a page. A Meta tag exists for this, and is used by some search engines when displaying the page as a result in SERP's.

A website that categorizes other websites. Such sites are usually manually compiled, depending on user submissions. Yahoo! And ODP are commercial directories, whereas Zeal is a non-commercial directory.

Also known as DMOZ. This is a very large directory run by volunteers. This database is used by many websites on the internet.

An optimized page submitted to search engines in order to rank high for very specific keywords. With the rise of link popularity as an integral part in ranking algorithms, this technique has become redundant. Such practice is considered spam by some search engines.

This is a web page that changes its content depending on database information or user settings. A scripting language such as PHP or ASP is usually used to bring about such a result. Search engines are now advanced enough to index such pages, but dynamic variables such as '?id' may cause problems.