Hi Shauna- Starting Tableau 10.5 you can actually downgrade the workbooks to as far as 10.2 version. So if you develop a workbook in 10.5 and "Export version as 10.2/10.3/10.4" you'll be able to open in either of those versions of Tableau Desktop. Coming to you next question, once you develop and dashboard and publish it to Server it would be available for sharing. You'll also have an option to "Turn On" Web authoring through which server users will be able to connect to published data sources and create workbooks online for ad-hoc analysis. Note that, Tableau online however lacks some key functionalities that Desktop offers. Desktop is best for full-fledged development and online is more of self-service analytics. Hope this helps.