Tax liens are usually for unpaid property taxes. These are senior liens on a property, meaning that they are generally the first to be paid when a property sells. If you are purchasing either a property that has completed foreclosure already, or in any other "normal" sale, the escrow company will arrange to pay and clear the tax lien at close of escrow.
A tax lien happens when a homeowner has not paid his property taxes. A lien is placed against the property and it has to be paid from the proceeds when the home sales. The state cal also move to foreclose on a property for unpaid taxes.