What are the differences between index databases and property databases in SharePoint2010 search?

What are the differences between index databases and property databases in SharePoint2010 sear开发者_开发知识库ch?

http://technet.microsoft.com/en-us/library/cc678868.aspx

Search service application Crawl database

The Crawl database stores the state of the crawled data and the crawl history.

Search service application Property database

The Property database stores information that is associated with the crawled data, including properties, history, and crawl queues.