Organizations vs Groups vs Channels

Trying to figure out when to use these types of content. How are groups and organizations the same or different? Are organizations more like a business card, or a fully functioning content area? (I can't get my site to allow me to add orgs so I haven't been able to try them). Are groups like subsets of the overall site? Do events/albums/posts etc added to the group appear to people not in the group? 

For my site the groups will be the most active areas, but I'm using this to end having more than a dozen separate separate site. The site wide areas will be designed to replace an existing Facebook private group.