The List Throttling is a brand new concept in sharepoint 2010.  The concept of this functionality is to set the limit on the row of data can be retrieved from a SharePoint List at on time. This concept is same for Sharepoint document library for retrieving data.

Use Case: The list throttling setting will be applied on list views created by users and queries executed by custom code. When someone runs the query to retrieve all items, the number of items returns will be determined by the throttling setting for the given list and the user rights.

Throttling Settings: You can set the throttling settings via central admin.

In order to go to these settings, follow this path, Central Administration > Application Management  >  Manage Web Application. Once in the Web Application List, select the web application and click General Settings > Resource throttling.