Crawl error may occur when search engine bot crawls your site. There are some type of crawl errors such as HTTP errors, Not Found, URL timed out. I think this link will help you http://www.google.com/support/webmasters/bin/answer.py?answer=35120
You need to check these errors with the help of webmaster tools and fix it. It may affect search engine rankings.
search Engine Bots will crawl your web site in certain periods and it will be different for each web sites. The web site which has a good PR will crawl more time than web sites having low PR. This crawler is a program running on certain algorithms and it will go through each pages of your web site. If the web site structure is good what it means is web site having less images, more texts, less flash, javascript, sign in forms, web sites behind forms, good scripting, css then Google bots will come to your web site and will crawl on specific times. In these crawling time Google will check whether web site has Http errors, page not found errors, duplicate meta-tags errors, duplicate content check, path not found, images not present, robots.txt format incorrect, sitemap.xml errors. If so it will detect and notify. So these crawling error u can find when u sign up for Google web master tool account and added web sites in it. If the craw rate errors are more then your PR will decrease and it will affect overall functioning of web site.
Which means Google Spider can Not access to the page URL they have indexed before. That might be caused by internet connection timeout, server down or page url deleting.
The Crawl errors page provides details about the URLs in your site that we tried to crawl but could not access. The Mobile crawl errors page provides details about problems we encountered crawling URLs on your mobile website.
Crawl errors can be usually found out through google websmaster tools.It may occur due to HTTP errors or URL timed out or server uptime problem.
Search engine bots come and crawl a website is called crawl error. Following error is the crawl error 1-Page Not Found 2-URLs Not Allowed 3-URL Unreachable