BSDF is a superset and the generalization of the BRDF and BTDF. The concept behind all BxDF functions could be described as a black box with the inputs being any two angles, one for incoming (incident) ray and the second one for the outgoing (reflected or transmitted) ray at a given point of the surface. The output of this black box is the value defining the ratio between the incoming and the outgoing light energy for the given couple of angles.
BSDF includes and generalizes BRDF and BTDF. All BxDF functions can be conceptualized as a black box that accepts any two angles: one indicating the direction of an incoming ray and the other showing the direction of an outgoing ray at a surface point. The output of this black box is the ratio of incoming to outgoing light energy for these specific angles.