Anyone know about Tax Liens?

I am trying to get a sense of just how much effort is involved in Tax Liens and just what can someone get out of it. For example, just how much research should be done before puchasing a lien. Do you have to go to the auction or can you buy them online? Do you have to monitor them or just wait for the check in the mail. I guess the real question is: how does this compare to a CD or mutual fund?