What are the differences between index databases and property databases in SharePoint2010 search?
In order to learn SharePoint 2010 Search Terminology, please check-out http://www.microsoft.com/downloads/en/details.aspx?familyid=920E8C04-E8A6-4079-8B17-F3FB070FDF0D&displaylang=en
Here is an additional link: http://technet.microsoft.com/en-us/library/cc678868.aspx
The index database is called "Crawl database" it stores the state of the crawled content together with the crawl history. In the "Property database" you will find metadata related to that content (like permissions and properties).
As a recommendation (if you plan to scale out): Separate property and crawl db's to their own storage to lower I/O load. In case you are crawling and quering at once, there is a heavy load on the sql servers. Each query action will rely hefty on the property db- a parallel full crawl that runs for hours or even days will bring your sql under pressure.
If you can do your full crawls at night and let the users query over the day, you should be fine without the seperation of the db.