Crawlers, spiders, search engines, I'm confused. Let's start with Search Engines; there are two types of search engines, crawler-based search engines and human-powered directories. The term "search engine" is often used in a generic way to describe both. These two types of search engines gather their listings in radically different ways.

Crawler-Based Search Engines
Crawler-based search engines, such as HotBot, create their listings automatically. They "crawl" or "spider" the web, then people search through what they have found. When you change your web pages, crawler-based search engines will eventually find these changes, and that can affect how you are listed. Page titles, text on your website the body copy and other elements all play a role.

Human Powered Directories
A human-powered directory, such as Yahoo, depends on humans for its listings. You submit a short description to the directory for your entire site, or editors write one for sites they review. A search looks for matches only in the descriptions submitted. Changing your web pages has no effect on your listing. Things that are useful for improving a listing with a search engine have nothing to do with improving a listing in a directory. The only exception is that a good site, with good content, might be more likely to get reviewed for free than a poor site.