What is Tableau Desktop? or Which software I need to use for Tableau application development?

Tableau Desktop is tool used for developing Tableau applications. It comes in three different versions, below table contains feature comparison of those. Though these details are directly taken from Tableau portal, for some reason I felt that it should be there in my portal.

** Pricing details are as of the post written date, for latest pricing please log on to http://www.tableausoftware.com/public/comparison.