The Admin API is a collection of calls that deal with system resources in the NICE inContact and Customer Interaction Cloud system such as Agents, Skills, AddressBooks etc...
The Agent API is used to create, manage, and end "agent sessions." With an agent session, you are able to manage interactions on channels such as Phone Calls, Chats, Emails, Voice Mails, SMS, and Work Items.
The Authentication API is a collection of API calls that deal specifically with the system resources required to login to the CXone platform.
The Patron API can be used to create patron-facing applications. For example, you can create a mobile application that allows a patron to request a call back or a web site that allows users to request a live chat session.
The Real-time Data API is a collection of API calls that provide access to "real-time" data on the platform. This API can be used to create custom dashboards and control panels, and to provide information to agents (leaderboards, etc.).
The Reporting API is a collection of API calls that provide access to "historical" data on the platform. This API can be used to run Custom Reporting reports and retrieve calculated metrics.