AWS Organizations have a Management ( also called Master or Main) account and Member Accounts.

Members account can be created, migrated from other organizations, or simply invited.

All this actions can be accomplished via UI console but, when numbers are large, it is very handy to be able to do these via Organizations API. 

AWS Organizations has 2 features set:

All features – The default. Basically the main point of entire service. ( especially tagging policies

Tag policies to enforce Tag Standardization across OU and Resources and SCPs)

Consolidated Billing – a subset of features, which provides basic management tools to centrally manage the accounts.