  The mechanism responsible for the afterglow emission of Gamma Ray Bursts
(GRBs) and its connection to the prompt $\gamma$-ray emission is still a
debated issue. Relations between intrinsic properties of the prompt or
afterglow emission can help to discriminate between plausible theoretical
models of GRB production. Here we present an overview of the afterglow and
prompt-afterglow two parameter relations, their physical interpretations, their
use as redshift estimators and as possible cosmological tools. A similar task
has already been correctly achieved for Supernovae (SNe) Ia by using the peak
magnitude-stretch relation, known in the literature as the Phillips relation
\citep{phillips93}. The challenge today is to make GRBs, which are amongst the
farthest objects ever observed, standardizable candles as the SNe Ia through
well established and robust relations. Thus, the study of relations amongst the
observable and physical properties of GRBs is highly relevant together with
selection biases in their physical quantities. Therefore, we describe the state
of the art of the existing GRB relations, their possible and debated
interpretations in view of the current theoretical models and how relations are
corrected for selection biases. We conclude that only after an appropriate
evaluation and correction for selection effects can GRB relations be used to
discriminate among the theoretical models responsible for the prompt and
afterglow emission and to estimate cosmological parameters.
