The Poke Tool is for manipulating the current values associated with components. The precise behavior of the Poke Tool varies depending on which component is clicked; this behavior is documented in the `Poke Tool Behavior' section of each individual component. The following components all have support for the Poke Tool.
Also, clicking a wire segment using the Poke tool displays the value currently carried by the wire, as described on the Wiring Tool's page.
None. Clicking on a component supporting the Poke Tool, though, will display that component's attributes.