javascript:void(0);

just mean do nothing : http://www.2ality.com/2011/05/void-operator.html

you ask to evaluate 0
void return nothing

This prevents an actual <a href="" /> from firing it's href value. That means the page won't be refreshed, redirected, etc. It forces the javascript methods (i.e. onclick) to be fired.

http://www.tizag.com/javascriptT/javascriptvoid.php

So you can have a bunch of different links get processed in one central function and decide what to do based on the source of the click, vs hard coding the href for each link?

no, the void is placed like so:

<a href="javascript:void(0);" onclick="DoSomething()">Click Me</a>

The void(0); tells the browser to NOT evaluate the href attribute value. Instead, perform any javascript events on the link.

If you only had the following, it would do NOTHING when the link is cliked:

<a href="javascript:void(0);">Do Nothing</a>

no that's not the purpose, people use # instead javascript:void(0)

the link stay a link (mouse over effect, underline, ...) but when you click on it nothing happen
most of time you have the onclick attribute set for the link

you can use : onclick="return false;" too

So what might be the reason for doing this?  A practical application?

For example, when I visit an answered question on EE, at the top it has links to "share the solution" on facebook, twitter, etc. and as I hover over each button (each link) it says:

javascript:void(0);

thx