You might want to tie a quilt when you’re using extra thick batting, thin or no batting, or whenever a quilt can’t be quilted the traditional way. It also makes for a quick and easy finish.
Yarn is often used for the ties, but any thick sturdy thread, such as several strands of embroidery floss, will work nicely. Cut the knotted yarn short or long.
Tying also can be used to affix buttons or charms to a quilt, adding another layer of interest.
A grid pattern works nicely, with the ties spaced according to what is specified by the batting. Just keep in mind that a quilt with closely spaced ties will wash and wear better than a quilt with the ties far apart.
Angela Mitchell provides a good overview of the tying process in the following article.
An excellent tutorial from New Quilters goes into further detail and provides many helpful suggestions.