A B-spline surface is defined by a control mesh. The surface follows it, just like a B-spline curve follows its control polygon. You may think of a B-spline surface as being made by stretching and shrinking an infinitely flexible rectangular rubber sheet over the control mesh.
The surface has two directions, called the cross-wise and lengthwise directions. The control mesh always has a fixed number of control points in its two directions. You may shape the surface by moving control points, you can add an entire row of control points, but you can neither add nor remove a single point at a time. In other words, the B-spline surface is composed of a rectangular array of surface patches.
The B-spline surface is able to assume a variety of shapes that do not appear to resemble the rectangular patch structure at all. For instance, a sphere may be made and the ‘rectangular’ patches become like lines of longitude and latitude.