What does Unity Multiplayer consist of?

Symptoms

•  I would like to know what the Unity Multiplayer framework in Unity is made up of

Cause

I would like to know how Unity Multiplayer is structured and how I can expect to utilize it for my project. 

Resolution

Unity Multiplayer contains several different facets:

1. High Level APIs (HLAPIs) makes it easy easy to implement and prototype networked features.  
2. Low Level APIs (LLAPIs) let you optimize network code for your game.
3. Matchmaker Server helps your players find game sessions so they can play with each other.          
4. Relay Server routes traffic between players to ensure reliable, stable connections that would otherwise be hampered by point to point connections (NAT, firewalls, etc).