ETL as I see it would be a sequence of steps that process or reshapes the data to help a particular analytic requirement. There are different ways of achieving this on Tableau - either within the data connections, or on the metadata layer. Data Extracts are saved subsets of the data source and could be a result of such ETL definitions. The options available for extraction are described in the online help document here - http://onlinehelp.tableausoftware.com/current/pro/online/en-us/help.htm#extracting_data.html
As for your final question, can you share what specific ETL requirements you are looking to address on Tableau 8?
Hope it helps