It is important to understand liens in general before moving on to the subject of tax liens. Black's Law Dictionary defines a lien as "a legal right or interest that a creditor has in another's property." When you take out a mortgage to purchase a home, you are voluntarily placing a lien against the property; the lienholder is the lender, the security is the house.
When you renovate your office building, the general contractor is responsible for paying all of the subcontractors. If the general does not, though, those companies may file liens -- specifically, mechanic's liens -- against your property. You may never have had any dealings with the plumber or electrician, but your property is the tangible evidence of their work.