Applies to SharePoint 2013
XSLTListview webpart was release with SharePoint 2010 to replace the Listview webpart which was used to display the list data in SharePoint 2007. It uses XSL and users can customize a lot of the XSLT and modify the look feel functionality which the LIisview webpart could not do. XSLTListview webpart works with lists and library but not with external lists.
Dataform webpart has always been around and is used to display data from list, library and external data sources as well.
Both dataform webpart and XSLT list view webpart can be customizable using XSLT for all sorts of things including Conditional formatting, sorting, filtering, grouping etc. SharePoint Designer is smart to user the right one depending on what you need it for and this is totally transparent to the user. When inserting a dataview in the page from Designer by selecting Dataview\ name of the list in the Ribbon, XSLT LIstview webpart is used whereas when Dataview\ Empty Dataview is selected from the ribbon in the designer, DataForm Webpart is used.
Check out my below posts on DataView Webpart, leave me a comment if you find them useful