  The space of light rays $\mathcal{N}$ of a conformal Lorentz manifold
$(M,\mathcal{C})$ is, under some topological conditions, a manifold whose basic
elements are unparametrized null geodesics. This manifold $\mathcal{N}$,
strongly inspired on R. Penrose's twistor theory, keeps all information of $M$
and it could be used as a space complementing the spacetime model. In the
present review, the geometry and related structures of $\mathcal{N}$, such as
the space of skies $\Sigma$ and the contact structure $\mathcal{H}$, are
introduced. The causal structure of $M$ is characterized as part of the
geometry of $\mathcal{N}$. A new causal boundary for spacetimes $M$ prompted by
R. Low, the $L$-boundary, is constructed in the case of $3$-dimensional
manifolds $M$ and proposed as a model of its construction for general
dimension. Its definition only depends on the geometry of $\mathcal{N}$ and not
on the geometry of the spacetime $M$. The properties satisfied by the
$L$-boundary $\partial M$ permit to characterize the obtained extension
$\overline{M}=M\cup \partial M$ and this characterization is also proposed for
general dimension.
