These are all the 400 series errors. So it includes 404s and 400, 401 etc. Here is a list more can be found at Wikipeida.org

400 Bad Request
The request contains bad syntax or cannot be fulfilled.

401 Unauthorized
Similar to 403 Forbidden, but specifically for use when authentication is possible but has failed or not yet been provided. See Basic access authentication and Digest access authentication.

403 Forbidden
The request was a legal request, but the server is refusing to respond to it. Unlike a 401 Unauthorized response, authenticating will make no difference.

404 Not Found
The requested resource could not be found but may be available again in the future. Subsequent requests by the client are permissible.
405 Method Not Allowed
A request was made of a resource using a request method not supported by that resource; for example, using GET on a form which requires data to be presented via POST, or using PUT on a read-only resource.
06 Not Acceptable

407 Proxy Authentication Required

408 Request Timeout
Client failed to continue the request

409 Conflict

More Info found here:

http://en.wikipedia.org/wiki/List_of_HTTP_status_codes

Is there a password protected area of your site, maybe bots are trying to hit that.

If you need any more information, or what me to analyse some of the possible errors drop by at www.wiredvillage.ca.

Good luck!
Source(s):
wikipedia.org

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RFC explaining all HTTP code can be found here: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

HTTP 4xx errors usually indicate that the server could not understand the request or cannot process it, the request was forbidden, or the request timed out. They also may indicate authentication error.
Source(s):
RFC2616

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This simple means that when google bot tried to access that page of yours, it got an error. Now that error could be because the coding is wrong on that page, or your hosting server is down, or your URL is not search engine friendly. I'd suggest have a look at it and if you find error is ffrom your side, correct it ASAP.
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The first thing to do is take a look at your raw log files to find what the specific errors are. The most worrying would be if you're continually getting a 404 (not found) error on those urls. This would mean that you have broken links on your site, in people's bookmarks, or on sites pointing to you. You can find the source of this error by looking at the "referrer" in the log file - i.e. where your links are coming from.

Fixing the problem depends on the referer. Possible solutions are:

- put in a redirect (if people have bookmarked old/moved pages)
- contact referring sites (if they are sending bad urls your way)
- fix your own content (if you've put in bad links)
- fix your content management system (if it's automatically putting in bad links)
- fix your sitemap (if you've registered one that contains bad links)

All of the above only apply to 404 errors. If it turns out the problem isn't 404 I'd suggest reposting with the exact error number from your log file.
Source(s):
http://en.wikipedia.org/wiki/HTTP_404

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