When we look at the world of the top search engines, it can be divided into Crawler-based, Human-powered, Hybrid, and Pay-per-performance search engines. Crawler-based or “traditional” utilize special software, and surf the Web to supplement their database. Search engine spiders scan parameters which are 
not visible to the human eye, and they don’t differentiate the amenities of design, but see only the code for images, which throws obstacles between it and your content. The most important data for search engine robots is the relevancy of your printed text, the frequency of your keywords and phrases in the contents of the page, especially at the beginning and the end of it, which should be emphasized. The top search engines of “traditional” kind are Google, Teoma, AllTheWeb, Alta Vista, and Hotbot.
Directories, which are also known as human-powered or edited search engines, do not look for websites to index them, but they rather prefer you take the action to suggest your site, and most of them have “Add URL” pages where you fill out the required information such as the website title, description, keywords, and email. You simply select the proper category for the website and then, in accordance of turn, it will be examined by human editors, leaving the decision of this editor only to accept or not to accept the site. So it is that human-powered search engines do not have their own algorithms to rank websites, they rank according to alphabetic sequence or Google Page Rank, and 
Directories are often used by top search engines as a source of new pages to scan for indexation, and finally, among the largest top search engines of human-edited nature there are Yahoo!, DMOZ, and Looksmart.
Hybrid Search engines utilize both types of listing, for example, MSN presents human-edited listings from Looksmart and crawler-based listings from its own Web crawler. Currently, some top search engines also resort to opposite listings such as Google and Yahoo, but it only takes a small part of the whole process, and they are still leading their classes. Also, there are meta-search engines, which work to combine the results from the number of other top search engines at the same time and present the results, and among this kind of search engines is MetaCrawler and Dogpile, for example, MetaCrawler compiles the results of seven search engines including AltaVista and Lycos.
In Pay-per-performance search engines, you pay for your site to be listed, re-spidered, and top-ranked according to keywords you both choose and bid on. It is important to note that there is not a big amount of sites focused only on paid inclusions, with the most prominent being Overture, and in addition you can find some additional paid services provided by other top search engines, but don’t feel obligated to use them.
