A shelf is an AREA of the visualization display where parts of information can be dropped to enable Tableau to act on them. If you place a dimension on the shelves of the rows or columns, headers are created for the Members.  

Tableau shows information with marks that match each mark in the information source to a line (or a set of rows). A shelf is a display area. There are four kinds of shelves:

• Columns
• Rows
• Pages
• Filters

The columns shelf creates the columns of a table, while the Rows shelf creates the rows of a table. You can place an unlimited number of FIELDS on these shelves.

When you place a dimension on the rows or columns shelf, headers for the members of that dimension are created. When you place a measure on the Rows or Columns shelf, quantitative axes for that measure are created. As you build up your data view with more fields, additional headers and axes are included in the table and you get an increasingly detailed picture of your data.

In the view shown below, the members of the Customer Segment dimension are displayed as column headers, while the profit measure is displayed as a vertical quantitative axis.

Tableau displays data USING marks, where every mark corresponds to a row (or a group of rows) in your source.

Each Tableau WORKSHEET includes CARDS and shelves like Columns, Rows and Marks, Filters, Pages, Legends, etc. Build the framework of your visualization by putting the areas on the shelves or cards.

Increase the amount of detail and control by including or excluding information the number of marks in the perspective. Add context by color, size, shape, text, and detail to the visualization. Experiment with placing areas on various shelves and cards to discover the ideal way of viewing your information.