Joining is where you are able to combine data from the SAME data source, for example worksheets in an Excel file or tables in an Oracle database. You need a common field, also called a key, that tells Tableau how to join the data. You also need a key to do blending.
Blending is where you able to combine data from DIFFERENT data sources, for example a worksheet in Excel with a table from an Oracle database. This is very powerful because in the past you would typically need to have someone with the specific technical skills create a new data set with the combined information.
