This is wildly out-of-date, both these hooks are deprecated. Instead you use ResourceLoader to load JavaScript resources which typically use jQuery to run code at the appropriate event.

(and yet some freewebhosts still only has old php, and we can only use old version of mediawiki)

addOnloadHook()

addOnloadHook(someFunction);

This hooks onto the completion of the page load, and runs the specified function. You can run an anonymous function too:

addOnloadHook(function(){
    ...
});

addHandler()

addHandler(domElement,'action',someFunction);

This hooks onto the action being performed on some DOM element, running the function. This can be useful for example if you want to change the user's input when submitting a form:

function reallyChangeInput(){
    ...
}
function changeInput() {
    form = document.getElementById('editform');
    if (!form) return false;
    addHandler(form,'submit',reallyChangeInput);
}
addOnloadHook(changeInput);