In the tools dialog, the following actions are available:

New
Shows the tool properties dialog where the properties of a new tool can be entered.
Edit
Shows the tool properties dialog where the properties of the highlighted tool can be edited.
Delete
Removes the currently highlighted tool.
Cancel
Discards all changes and closes the dialog.
OK
Saves all changes and closes the dialog.

The definitions of the tools are written in the desktop configuration file. So unless auto-saving of the desktop file is enabled, the desktop file should be saved explicitly after the dialog is closed.