Tableau tool is used where we have huge data and you want to take out the insights from the data. So manually it is not possible to do the calculation and other things, that’s the place where BI comes in to the role. If we can make some interactive visualizations out of it then it will be easy to understand the insights unlike the spread sheet data.
Tableau Desktop helps to understand the data in few minutes, professional version of Tableau can enable to transform, pre-processing and store huge volumes of the data locally and is responsible for data driven decisions making by the organization or company. By using Tableau desktop you can save and share the visualizations to others (You give access to those). Other than those customers no one can see your visualization.
Tableau public didn’t have facility to save the data locally, Other than this if you publish the report then it can be seen by anyone. That means there is no privacy while using Tableau public. Tableau public is a free tool and can be used for learning or practice or where you didn’t have security issue.
If you want to learn Tableau Desktop then you can check learn Tableau.