  Consider two families of closed oriented curves in a d-manifold. At each
point of intersecction of a curve of one family with a curve of the other
family, form a new closed curve by going around the first curve and then going
around the second. Typically, an i-dimensional family and a j-dimensional
family will produce an (i+j-d+2)-dimensional family. Our purpose is to describe
mathematical structure behind such interactions.
