The reason for using cloak technology is twofold. First, because viewers never see the page that is presented to the spider, the code can not be stolen. In highly competitive markets the ability to conceal code from the prying eyes of competition can be an seen as an advantage.

Second, since the page that is served to the spider is never seen by a human visitor, it can be crafted in such a manner as to explain specifically what that page is about - which may be very useful depending upon your design. For instance, it's often seen as a good solution for sites that have been built totally in Flash - as the search engines can't read that technology.  That said, it's still not worth getting your website blacklisted.

Other search engine terminology to be familiar with....

Blacklisted: How to tell if your website has been blacklisted or un-included in the search engines.  On Google go to Google.com and type in site:www.whatever_your_domain_is (filling in the domain name, of course).  Any other search engine, just type in the search bar www.whatever_your_domain_is (filling in the domain name, of course).  If your website isn't listed, then the search engines are either slow at listing your or you have been blacklisted or removed from the search.

Click popularity: a measure of how often a listing presented by a search engine is clicked. Some search engines and directories will rank a site higher on their results pages if the site proves to be popular among searchers.

Cloaking: also known as "stealth," involves serving a specific page to each search engine spider and a different one to human visitors. In most cases, frowned upon by search engines.

Comment tag: html code that describes or documents content. Most search engines ignore the content of comment tags.

Directory: directories are built from submissions made by website owners, and generally arrange site listings hierarchically. Yahoo! is the best known example.

Doorway page: a web page created solely to achieve high ranking in search engines for particular keywords, and perhaps for a specific engine. Today's doorway pages should contain valuable and useful content related to your site, and be fully linked to the site, and so are often referred to as "information pages."

Dynamic html: web pages generated on demand by data in databases or using similar technology. Can create ranking problems because a search engine's spider may not retrieve relevant content.

FFA Site: A so-called "free for all links" page, which is created for the sole purpose of compiling links. Submission software or companies that claim to submit your site to hundreds or thousands of "search engines" actually use these for most of that number. FFA sites are essentially worthless in terms of generating traffic, and links from them will count nothing towards your site's link popularity. Basically, they're a waste of time.

Frames: using frames allows the division of a visible web page into separate scrolling regions. Can cause both navigational and ranking problems as search engine content.

Hidden text: making your text the same color as your background rendering it invisible to your visitors.  Your text appears to not be there but the search engines and human search engine editors can see it anyway.

Hit: relating to a search engine, the number of sites or pages that are returned in response to a given query. Relating to site traffic, a hit is counted each time an individual file (document, image, multimedia file, etc.) is served in response to a visitor's request.

Informational page: a content-filled web page created to focus on particular keywords. Differs from a "doorway page" in that is wholly integrated into the site and is useful to human visitors, while a traditional "doorway page" is aimed only at search engines.

Invisible text: using a font color the same or close to the color of the background of a page, in an attempt to allow the content to be indexed by search engines while not being visible to humans. 

IP delivery or IP-based delivery: the technique of serving a particular page in response to a page request from a specific IP address. Used in cloaking; a search engine is identified by the IP address it is using, and a page customized for that search engine is served.

Keyword Stuffing: placing the same keywords over and over again throughout your text, meta tags and titles with the main purpose of attracting search engines to rank you highly for that keyword.  It's best to use relevant text to have naturally occurring keywords throughout your website based on good content.

Keyword density: the ratio of the number of occurrences of a particular keyword or phrase to the total number of words in a page. One element of search engine optimization.

Link popularity: essentially a measure of how many other sites indexed by the same search engine have links to your site. Link popularity is essentially simply a count of links to a particular site; see also link analysis.

Link analysis: a measure of the quality and relevance of the set of links pointing to a given site; contrast with link popularity.

Link Farms: sites created and maintained solely for the purpose of constructing links between member sites. Should be avoided as a violation of most search engines' policies; their use won't build your site's link popularity, and may result in a ranking penalty.

META refresh tag: automatically replaces the current page with a different one within the website, or possibly offsite. In general, use of refresh tags is discouraged or penalized by search engines.

META tag: html tag in the header section of a web page, intended to offer content to search engines. Among them are the keyword and description tags, but these days most true search engines de-emphasize or completely ignore META tags.

Relevancy: how closely related a particular page is to the search term requested.

Reputation: related to link popularity, a page will score highest for reputation when it is linked to by pages from other sites which themselves are highly ranked. Well-known sites recognized as "authoritive" are given high reputation scores on their own; it's for this reason that a link to your site from something like cnn.com would be very valuable.

Search engine: one of the internet's searchable databases of web pages, generally built by using "spiders" to locate and read pages.

Search engine marketing: encompasses several forms of marketing products and services on the internet through management of information presented by search engines and directories. Included are such elements as site optimization, and the purchase and placement of advertisements.

Search engine optimization: the process of developing web pages or web sites in such a way as to achieve favorable placement in search engines and directories.

Search engine positioning: the process of managing a page or site's positioning in the search engines.

Selective delivery: the technique answering browser's page request with a specific page selected via an automated process based on some piece of information gained from the browser. For example, reading the browser's language setting may allow a page in that language to be served. Similar to IP-based delivery.

SERP: A "search engine results page," the page of site listings that a search engine returns in response to a user's entry of a search query. Often used in discussion of the way such a page is laid out, for example: "Overture listings are the first sites presented on Yahoo's SERPs."

Spam: as it applies to search engines, any attempt to submit or place deceptive information, or to "trick" the search engine into placing a page in an inaccurate position.

Spider: a program that traverses the web, following links from page to page. Also called a robot.

Stop word: common words, or words considered by search engines to be irrelevant, are left out by search algorithms. Examples are "and," "the," etc. Generally, a stop word in a query is treated as a "wild card;" that is, the returned results usually won't be exactly the same as if the word had been left out of the query entirely.

Theme: a relatively recent change in search engine ranking algorithms, theme-based engines essentially try to determine what a page is "about" — and to compare it to other pages that seem to be related to the same topic — and rank it highly for certain keywords that are determined to be related to that page theme.

Word stemming: a practice used to some search engines in which searches will return results for words based upon a particular stem. For example, a search for "develop" might return pages containing the words "development" or "developer.