A work item is a transaction you are actively working. Inbound calls, call backs and SMS are all work items. If you receive a voice item, it will automatically put you in "Working" Status, you will have to complete this item before you will be able to receive another call. 

If you receive an inbound SMS, you will show "Working" but you are still available for calls. You can leave SMS workitems open until you have completed them, or transferred them back to the queue.