These levels are divided according to what members of each level are able to do.
A team on Flood is similar to a security group, and is the smallest bucket available. There can be multiple teams within an organization, and you can also nest teams (sub-teams) within teams. An example of how you could set teams up is to have one for each level of access you'd like to grant people, such as Team Admins, Flooders, and Guests.
The next level up is the organizational level. There can be more than one organization for each account. You could use this level for logical groupings of teams within your company. For instance, you could create an organization for each product.
The highest level is that of your Flood account. All activity on the same account is billed on one invoice.