I am learning much from the Simply Javascript book - now in the chapter about events. I have a simpleminded question - when you call an isolated funcition you use parenthesis after the name of the function: e.g. dothisfunction()

It seems that when the funtion is a method inside of an object you leave out the parenthesis - quoting from page 108 of the book I see:
link.onclick = wikipediaLink.clickHandler;

rather than:

link.onclick = wikipediaLink.clickHandler();

Am I correct and can anyone explain why?

Many thanks for any clarification.