Three spectroscopic groups (A, B, C) of RV Tauri stars are identified from a study of low-dispersion spectrograms of 48 stars that are classified as RV Tauri stars or yellow semiregular variables in the second edition of the General Catalogue of Variable Stars. Except for the TiO bands that may occur during deep light minima, all spectroscopic features of members of group A indicate spectral type G or K. The stars in group B show a discrepancy at all phases between the spectral type based on the Ca ii lines and that based on the hydrogen lines; between secondary and primary light-maxima, strong bands of CN are present. Members of group C resemble those of group B except that bands of CN are never seen. The RV Tauri stars can usually be distinguished from the SRd variables on the basis of the strength and duration of the hydrogen emission. Concurrent U, B, V photometry of 15 of these RV Tauri stars indicates that the members of group A are systematically redder than those of groups B and C. The latter two groups appear to be indistinguishable photometrically. The U - B and B - V color indices of the RV Tauri stars at most phases lie near the locus of normal, non-variable supergiants in the U - B, B - Vplane; the color indices of the semiregular variable SX Her deviate markedly from this locus. Systematic differences occur in the phase shifts between the V, B - V, and U - B curves of group A and groups B and C. It is suggested that the yellow semiregular variables and the members of group A constitute two relatively homogeneous families of variable stars. The status of groups B and C is uncertain. Finally, a recent binary interpretation of EP Lyr, a member of group B, is discussed.