Traditionally lariat necklaces are open-ended and very very long. Long enough to wrap around your neck two or more times. They are open-ended so that you can leave them hanging free or knotted in different ways to suit your mood and style. Sometimes they may also feature a slide on the front to vary the length when worn.
These days, you will find a number of necklaces that are also called lariat necklaces but which are much shorter and usually secured at the back with a clasp. They still feature the lariat-look -- having a knotted or clustered center in the front, with decorative dangle or hanging tassle hanging down the front.
These necklaces are also sometimes called Y-necklaces.