A custom object's setup page has a section "Buttons, Links, and Actions". I've created buttons and links in the past, but do not remember the "action" option being there.
Can anyone explain what "action" is for and compare it with what "button" offers? The salesforce help doesn't seem to have documented "action" yet.
These relate to a new feature to extend Chatter in Summer'13, allowing you to add your own actions for users to select when responding to comments left directly in the Chatter feed.
I've seen them called Multi Dimensional Publisher (MDP) Actions or Publisher Actions or Chatter Actions. The branding is I think still settling. It is odd the standard links are broken, its worth submitting a case to them to fix this. By searching for Publisher Actions in the main help I found this page that should lead you to the main topics you need. Also the Summer'13 training docs have a good video here. You can also find an excellent blog on the topic here from Daniel Hoechst (Salesforce MVP).
For a more advanced discussion on using Visualforce pages with these button see here.
