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 encompasses and generalizes BRDF and BTDF. All BxDF functions can be conceptualized as a model that takes two angles: one for an incoming ray and another for an outgoing ray at a surface point. The result is the ratio of light energy before and after interaction, defined by these angles.