Difference between System.Web.UI.WebControls.WebParts.WebPart and Microsoft.SharePoint.WebPartPages.WebPart?

Microsoft.SharePoint.WebPartPages.WebPart is provided in MOSS 2007 to provide backwards compatability with MOSS 2003 webparts.

 In MOSS 2007, it is recommended to use System.Web.UI.WebControls.WebParts.WebPart instead.

System.Web.UI.WebControls.WebParts.WebPart does not provides a feature to get or provide to to other webparts.