Command links can be used to:
To get an idea of the range of commands available, look at the General > Keys preference page. This page is used to bind key sequences to commands so it shows the list of available commands. Note that not every command will do something useful from help content. Many commands are designed to work within a somewhat narrow context (like a particular view, or expecting a certain type of selection).
If you want to define your own commands, refer to the documentation for the org.eclipse.core.commands API package and the org.eclipse.ui.commands and org.eclipse.ui.handlers extension-points.