I've been using both systems, though I have to say that I don't like Hydra as much.
Yarold's works like this:Once you have an account, you will add one or more links to your own link section (you need to have at least one active link for the system to work). Then, you click 'link exchange' where you'll be given a list of links of other users to click.
http://i52.tinypic.com/2d27s7d.jpgThen you click one link which opens in a new window. When you click the links one by one, they'll all open in the same window one after another (which is very pleasant). Some links require you to do something, e.g. click on something on the site (like KofK where you click the food/toy). On those sites, your click is only counted after waiting for a few seconds as the site will believe you that you did your duty after that.
http://i53.tinypic.com/2cyk51f.jpg -->
http://i56.tinypic.com/2h6ep3a.jpgAs you click, you gain credits, depending on the link (some give you more, others less). Now your link is shown to other users that can click yours which costs you credits.
How Hydra works:Hydra works in pretty much the same way, yet it has quite some additional features, including upgrades you can buy for credits.
http://i51.tinypic.com/2z7o9lg.jpgIt also has some sort of 'Visitor', yet I'm not quite sure how it works (as Yarold's has nothing like this); however, I believe it to be cheating.