Tatting is the art of making lace by tying half-hitches in one thread and transferring the knots to another thread.
As far as I know, tatting is unique in using two threads. There are many arts that use just one thread: knitting, crochet, netting, cleeking, nalbindung, needle lace, the nameless art done with a long hook and misclassified under crochet or knitting as "tunisian crochet" or "afghan stitch," and dozens that I've never heard of. There are many arts that use an indefinite number of threads: weaving, warp knitting, macramé, twining, bobbin lace and other forms of plaiting, and heaven only knows how many more. Tatting is the only art that requires exactly two threads to form a stitch.
Tatting is also unique in that it is strictly a method of making lace, and is not a satisfactory way to make solid fabric. Rings and loops are its fundamental structure. Other lace-making techniques are adapted from fabric-making techniques; even the plaited laces sometimes regarded as "real lace" are, in their simplest forms, solid. Crochet comes closest to tatting in its laciness, for in crochet, holes aren't created, but are simply not filled. Perhaps this is why crochet is sometimes confused with tatting even though the two laces don't resemble each other in any other respect.