Shape is represented with the  icon.

You can only use a Discrete field in Shape, and it works the same as if you a Discrete field in Color.

When you put a field in Shape, the different values of the field have different shapes. When you click on the Shape button, Tableau opens a new window where you can, as with Color, select predefined Shapes palettes and add them to the values. 

A really interesting aspect of Shapes is that you can add custom shapes. To do that, find the Shapes folder in your Tableau repository (usually inside the My Documents folder on your computer). In the Shape folder, there is one subfolder by palette. You can create a new folder (of the name of your choice) and put different images in it (PNG, JPG, BMP, or GIF). Then, in Tableau, in the Edit Shape window, you'll find your new palette with your custom Shapes (if the new shape doesn't show up, click on Reload Shapes). Here's an example of custom Shapes with country flags:

The next property exists for both Line and Polygon.