Custom Event's are events that can be used to invoke a program function. This can be found in the variables panel by selecting a variable and enabling the 'Invokes the Function?' option and then selecting "custom event".
Start
Only works when the variable is the world. This will invoke the function as soon as the graphic is loaded.
Timer
Only works with Timer Components. This custom event triggers everytime the timer goes off.
Initialize
This event would run on any component, and it would fire when the component is loaded. This is a good way to load things