The focal set of a surface is the locus of points of its centres of curvature. Generally, a (regular) focal set of surface may consist of two surfaces, so called focal surfaces or evolutes, then of one surface and one curve, or of two curves. In this thesis, differential geometric properties of the focal set are studied. In the special case, when the focal set consists of a surface and a curve, so called canal surfaces are defined. Also, their special classes are analysed: tubes and rotation surfaces. Finally, in the case when the focal set consists of two curve, we defined cyclides of Dupin of elliptic-hyperbolic type. They are named after the French mathematician Charles Dupin who studied them in the 19th century. This thesis presents all the definitions related to the focal set of a surface. All theoretical material is accompanied by concrete examples and figures made using the software package "Wolfram Mathematica" [7].