What's the difference between diamond-shaped and square-shaped nodes?

Hi Tom,

depending on the tool you use, the answer can differ ;)

If you are referring to the different node types that you can see when you use the node tool with a normal path, then the diamond shaped nodes are cusp nodes, where you can move both handles independently and thus get a sharp corner. And the square ones are either smooth or symmetrical (both handles are always aligned). The round ones are automatically rounded ones.
More info here: http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Paths-Bezier-Curves.html

Other tools:

The star tool, for example, uses a diamond-shaped node to set the spoke ratio of a star, and another one to adjust the base radius.
Other tools also use different node shapes to make it easy to differentiate between the type of change you can make by moving those.
See the manual section for the specific tool to find out more.

Regards,
 Maren