As
Webster 1913 points out, a lien a legal charge on personal or real property. There are typically two types of liens,
voluntary and
involuntary.
A voluntary lien is created when a property owner enters into a contract that puts a lien on a property, such as a first deed of trust.
Many other liens are involuntary, such as mechanic's liens on a house (liens for payment for a contractor or some other individual for work on your house) which can be very damaging to your title, property tax liens, (which are almost omnipresent on your title, above *all* other liens) and judgement liens, basically for when you fuck up in life and a judge decides that you have either too much stuff or too much equity in your home to be as dumb or mean as you are.
Another way of dividing up liens is the distinction between general liens and specific liens. A general lien is on any property owned by an individual in a given county and a specific lien is a lien on a specific piece of property.