Though the term "search engine" is used for most search tools on the Internet, not all search engines behave the same. Knowing the difference will help you in your quest.

Search engines
Use search engines when you know exactly what you want. Simply type the key words in the search engines dialog box.

When you type in a search term, your computer doesnt actually go out and search the entire Internet. What happens is you actually end up searching computers that have already searched the Internet. Before you ever enter your search term, the search engines computers automatically roam the Internet and index all the content on the Web.
Picking a search engine is very subjective. Currently, I use three search engines at most. But to pick my favorites, I tested well over a dozen. I'd like to say that my decision came from quantitative reasoning, but it didn't. I used qualitative judgement. Over time, I found more stuff faster using the following three search engines:

• Hotbot
• Google
• Fast Search

Directories
Use directories when you want to browse. The difference between search engines and directories has to do with navigation. Directories are usually easier to understand because they're manually created by humans, unlike search engines, which are automatically generated by computers. You begin with a general topic then step down one level at a time until you find the specific topic you are looking for.

The most popular directory is Yahoo!. In fact, most other search engines pick up listings from Yahoo!

Natural-language search engines
To use these search engines you simply need to know how to ask a question. If you've got a very complicated search, a natural-language search may be the answer. Instead of typing in search words, type in a full question (for example: "Where can I buy sandals?"). The most popular natural-language search engine is Ask Jeeves. Though the company has received lots of press on its "ease of use," Ask Jeeves can be frustrating due to its erroneous results.
Ask Jeeves is a great tool to use if you're hoping not to find what you're looking for. I think it stinks. But don't trust me, try it yourself. Search for a few sites you already know exist. I bet you'll have better success with Hotbot and Google than you do with Ask Jeeves. Let me know how your test works.

Category searches
If your search is more specific to a category such as medicice or law, try one of many category-specific search engines. A great place to find one is refdesk.com and LibrarySpot.

Meta searches
An online search usually involves multiple search engines. This can be a tiresome process. A meta search queries multiple search engines automatically. This makes them great for simple searches, but bad for complicated searches. The reason is that search engines don't have a unified language for accepting keywords. For example, some understand Boolean logic, some don't. Therefore, if you were to use Boolean logic in a meta search, it would confuse the non-Boolean search engines and return erroneous results.

Two of my favorite meta search engines are Dogpile and Metacrawler.