* Used to query set of items within a particular list
* Significantly faster to fetch items out of a list compared to SP object model.
* Takes CAML query as input to the query property and returns SPListItemCollection
* Used to query set of items from many different lists through the entire site collection
* Queries run through SPSiteDataQuery object are sometimes known as cross-site queries.
* Takes CAML query as input to the query property and returns ADO.NET DataTable object
* Scope of the query is controlled in CAML query using Webs property of the object.
Hope this helps!!!