As John implies, you really need to add risks to this list because that always seems to cause confusion as well. My thoughts are fairly similar to John's:
Task (in the context of a project) - an item in the WBS at the lowest level
Issue - an item that is impacting one or more tasks that requires immediate attention / resolution. An issue may result in new tasks being created, tasks changing (time, effort, priority, substance, etc)
Action Item - a specific action assigned to an individual to be completed by a certain time, often as the output of a meeting. The action item may contribute to completion of a task, resolution of an issue, etc
Risk - An item that has the potential of impacting the project - often by creating one or more issues if it is realized. High priority risks need active management through the creation of action items
Hope that helps!
Andy Jordan, President, Roffensian Consulting. andy.jordan@roffensian.com