Directories have a couple of disadvantages compared to search engines:

1. Directories are more limited in the portion of the Web that they're actually able to categorize, because all of that work is performed by people, who, unlike programs, need to sleep, eat, and spend time away from their computers.
2. Developing a classification schematic for the entire base of human knowledge is a fairly daunting, and subjective, task.
   There is no standardization between different Web directories' categories.
   One may have a category for "Daily Living" that addresses health issues, whereas another may have a section dedicated solely to "Healthcare."
   If your search strategy is to work your way through a directory's categories, it may be easier to stick with one directory once you get a feeling for how it categorizes information. This will save you time when you are looking for information, because, if you are familiar with how a specific directory categorizes information, you'll know where to look rather than spending time trying to figure out in which category it might be.

Each of the major search engines differs in some small way. Google is the king of search engines, in part because of the accuracy with which it can pull the results from a search query. Google offers all kinds of extras like e-mail and productivity applications, but those value-added services are not what made Google popular. What turned Google into a household word is the accuracy with which the search engine can return search results. This accuracy was developed when the Google designers combined 1) keyword searches with 2) link popularity. The combination of the keywords and the popularity of links to those pages yields a higher accuracy rank than just keywords alone.
However, it isimportant to understand that link popularity and keywords are just two of hundreds of different criteria that search engines can use in ranking the relevancy of web pages.

Most people assume that Yahoo! is a search engine, and it is. But Yahoo is also a web directory, which basically means that it is a list of the different web pages available on the Internet, divided by category and subcategory. In fact, what few people know is that Yahoo! started as the favorites list of the two young men who founded it. Through the acquisition of companies like

1. Inktomi,
2. All the Web,
3. AltaVista, and
4. Overture,

Yahoo! gradually gained market share as a search engine. Yahoo!, which at one time used Google to search its directory of links, now ranks pages through a combination of the technologies that it acquired over time. However, Yahoo!'s' link-ranking capability is not as accurate as the link-ranking capability of Google. In addition, Yahoo! also has a paid inclusion program, which some think tends to skew search results in favor of the highest payer.

MSN's search capabilities are not quite as mature as those of Google or Yahoo! As a result of this immaturity, MSN has not yet developed the in-depth link analysis capabilities of these other primary search engines. Instead, MSN relies heavily on web-site content for ranking purposes. However, this may have a beneficial effect for new web sites that are trying to get listed in search engines. The link-ranking capabilities of Google and Yahoo! can preclude new web sites from being listed for a period of time after they have been created. This is because (especially where Google is concerned) the quality of the link may be considered during ranking. New links are often ignored until they have been in place for a time.
Because MSN relies heavily on page content, a web site that is tagged properly and contains a good ratio of keywords will be more likely to be listed by the MSN search engine. So, though it is not the most popular of search engines, it is one of the primaries, and being listed there sooner rather than later will help increase your site traffic.