The Event System supports a number of events, and they can be customised further in user custom user written Input Modules.
The events that are supported by the Standalone Input Module and Touch Input Module are provided by interface and can be implemented on a MonoBehaviour by implementing the interface. If you have a valid Event System configured the events will be called at the correct time.
IPointerEnterHandler - OnPointerEnter - Called when a pointer enters the object
IPointerExitHandler - OnPointerExit - Called when a pointer exits the object
IPointerDownHandler - OnPointerDown - Called when a pointer is pressed on the object
IPointerUpHandler - OnPointerUp - Called when a pointer is released (called on the original the pressed object)
IPointerClickHandler - OnPointerClick - Called when a pointer is pressed and released on the same object
IInitializePotentialDragHandler - OnInitializePotentialDrag - Called when a drag target is found, can be used to initialise values
IBeginDragHandler - OnBeginDrag - Called on the drag object when dragging is about to begin
IDragHandler - OnDrag - Called on the drag object when a drag is happening
IEndDragHandler - OnEndDrag - Called on the drag object when a drag finishes
IDropHandler - OnDrop - Called on the object where a drag finishes
IScrollHandler - OnScroll - Called when a mouse wheel scrolls
IUpdateSelectedHandler - OnUpdateSelected - Called on the selected object each tick
ISelectHandler - OnSelect - Called when the object becomes the selected object
IDeselectHandler - OnDeselect - Called on the selected object becomes deselected
IMoveHandler - OnMove - Called when a move event occurs (left, right, up, down, ect)
ISubmitHandler - OnSubmit - Called when the submit button is pressed
ICancelHandler - OnCancel - Called when the cancel button is pressed