What is the difference between the different methods of putting JavaScript code in an <a>?

function DoSomething() { ... return false; }

<a href="javascript_required.html" onclick="doSomething(); return false;">go</a>

<a href="#" onClick="DoSomething(); return false;">link</a>

<a href="#" id = "Link">link</a>
(document.getElementById("Link")).onclick = function() { DoSomething(); return false; };

5: <a href="#" id="myLink">Link</a>

document.getElementById('myLink').onclick = function() {
    // Do stuff.
};