Unity has built in functions known as Event Functions that work on an event-driven framework. The Monobehavior class implements this event-driven framework with functions like Awake, Start, Update, etc. Also, all functions starting with On are part of the Event Functions, for example: OnCollisionEnter, OnGUI, OnMouseEnter, etc. The full list of methods can be seen here and the execution order can be seen here.
NOTE: Do not confuse Event Functions with the Event System or UnityEvents.