A judgement is a finding you owe money.
A lien is a method of securing (that means making it a part of the public record) that debt to something...generally property or something you could sell. It remains as a claim against the property (or the rights, or whatever). Hence, you can't sell it because anyone buying gets the responsibility of paying off the debt before they can get title or ownership.
A State or Federal tax lien just shows the debt originated from taxes.