Knot work is often attributed the Celts, however this style of design originated with the Picts and was transferred to the Norse before finally appearing in Celtic art ever so often. It was not until well after the christianization of Ireland that knotwork began to appear in abundance in Insular art, especially in manuscripts like the Book of Kells.
It is a typified by a series of interwoven lines, giving the work a slight three dimensional appeal. The sample above was chosen to demonstrate both styles of knot work, basic woven patterns like in the Shield Knot in the center, and zoomorphic designs like the grappling dogs on either side.
Knotwork is almost purely decorative and completely lacks any traditional symbology, contrary to the assertion of Neopagans and unscrupulous jewelry merchants who make up all sorts of claims of ancient meanings to sell jewelry. The only reason a knot work section is included here is because time and again I get the question "What does this knot mean?", and unfortunately with the exception of the Shield Knot, the answer is nothing.