The Different Types of Search
Search sites fall into two main categories:
Search Engines - which use robots or "spiders" to travel the web finding information, which is then indexed and ranked.
The best known Search Engines in the western world are Google, Bing (formerly MSN/Live), and Yahoo, although there are many others such as Ask Jeeves and the original search engine, Alta Vista.
Directories - which rely on cataloguing sites which have been submitted to them and which are then assessed - usually by human intervention
The best known Directories are DMoz and the Yahoo Directory although there are a great many smaller ones and many specialist one in fields such as business.
Search Engines rank sites in order of preference for the search terms that you give them, according to the various algorithms that each one prefers, and they each have "updates" - some are going on almost continuously while others may only happen every couple of weeks or more - where they re-rank the importance of the sites which they index. Every now and again there may be a major update, or algorithm shift, where they attempt to improve the basic ranking methodology which they use. Sometimes this is simply to reflect the research they have been carrying out while sometimes it is because their results are being permeated by poor quality sites which have found a loophole in the previous algorithm in order to get themselves ranked more highly than they deserve on their content.
Your ranking for any given search phrase will depend on how relevant the search engines believe your site is based on a number of parameters of varying importance. The number of different parameters that they use to rank the sites which they have in their index is sometimes very high. It is believed that Google uses around 200 parameters which may of course all interrelate. Some engines rely on the on-page content and how it is structured while others will also look at which sites links to yours and what link text they use to do so.
Getting into the search engines indexes is usually quite simple these days - just make sure there is a link into the site from another which is already indexed, and the spiders will quickly find the link and pay you a visit.
Paid Search Results
In addition to the normal "organic" listings most search engines also display "Sponsored" or "paid" results. These depend on the sites concerned having paid for a given amount of exposure related to the search terms that they are interested in. Google have their own system for this - Google Adwords - while other well-known agencies providing paid results are Overture and E-Spotting.
Directories adopt an entirely different approach. They seek to build up a structure based on subject groups, related topics and sub-topics that they believe reflects the things their users want to find. So for instance they might have a top-level which included Sport, which would be divided into various topics such as ball-games, raquet-sports, motor sports, etc. while in ball-games we might find football, rugby, basketball, volleyball, etc. Any site which was either selected initially or which was added later by either the directory themselves or suggested by their users, would be slotted into this structure, and if a suitable category didn't already exist then it could be created.
Getting into a Directory depends on being submitted - either by yourself if it is a paid submission, or by others - and then being reviewed and accepted.
Some directories may simply list the sites within their categories in alphabetical order, or perhaps in chronological order, while others may attempt to give them a ranking by some method - perhaps asking their users to vote for a site according to how good or bad they felt it to be. Many of the biggest and most important directories require a fee to be paid for inclusion while others have a multi-tier system where the more you pay the more prominently your site will be displayed. There may also be a free submission where there will usually be a delay in reviewing the site and it may not appear on the most visible pages. Other directories are entirely free and depend on advertising or sponsorship for their income.