This is a video by Michael of Google Webmasters Help on Crawl errors.
This video will help you to remove site errors and URL errors
View crawl errors
On the Webmaster Tools Home page, click the site you want.
Click Health, and then click Crawl Errors.
Site errors : Site errors are mostly server errors that occur mostly when your site server is down or web hosting is blocking by googlebot
URL errors : These are url specific errors that are related with specific pages of site, when specific pages of site aren’t working due to broken links