Active and Passive slots differ in the way they generate event (JMS message) for requesting the content to populate themselves.
Active Slot: A slot whose event generation option is set to "When Empty". In this case the slot itself can act as a scenario event i.e. it can issue its own requests for items. An active slot can fill itself when required.
Passive Slot: It is the slot whose event generation options is set to "Never". In this case the slot itself does not issue a request for content items by firing an event and can display items that are added to the slot through a scenario's "Add Item to Slot" element. The slot will not have any content if the scenario's "Add Item to Slot" element is not triggered.
If you have several pages which display the same slot then with passive slot, you would need to specify a page visit event in each page as a triggering event for the scenario used for populating the slot or run that scenario for all pages. With the active slot, you do not have to specify pages within the scenario because the slot itself can fire an event for requesting its items when any of those pages is displayed.
Example: Passive slot - user registers on site which causes registration event to be fired. A scenario detects and responds to the registration event and fills the slot with content say promotion.
Active Slot - user visits the home page which has a banner slot. If slot was having no content then it will request for it by firing an event and a scenario responds to event and fills the content in slot.
The explanation may sound bit confusing but it will become more clear once you use these options and actually see everything in action.
Passive slots :
-- It does not generate event
-- when user visits page, slots display the content that are currently filled in that slot
-- we can say it is static -- it shows what it is filled with
Active slots :
-- Active slots however act as a scenario event
-- scenario define what content should be their in the slot and slot displays that content
-- so we can say it is dynamic
-- When user visits the page an event is generate
-- this act as a scenario event and triggers items to fill in the slots