The crawl log tracks information about the status of crawled content.
The crawl log lets you determine whether crawled content was successfully added to the search index, whether it was excluded because of a crawl rule, or whether indexing failed because of an error.
The crawl log also contains more information about crawled content, such as the time of the last successful crawl, the content sources, and whether any crawl rules were applied. Use the crawl log to diagnose problems with the search experience.
For more information about crawl logs and search diagnostics, see TechNet: http://go.microsoft.com/fwlink/?LinkId=261561.