  Employing {\it ab initio} electronic structure calculations we extensively
study ternary Heusler compounds having the chemical formula X$_2$X$^\prime$Z,
where X = Mn, Fe or Co; Z = Al or Si; and X$^\prime$ changes along the row of
4$d$ transition metals. A comprehensive overview of these compounds, addressing
the trends in structural, electronic, magnetic properties and Curie temperature
is presented here along with the search for new materials for spintronics
applications. A simple picture of hybridization of the $d$ orbitals of the
neighboring atoms is used to explain the origin of the half-metallic gap in
these compounds. We show that arrangements of the magnetic atoms in different
Heusler lattices are largely responsible for the interatomic exchange
interactions that are correlated with the features in their electronic
structures as well as possibility of half-metallicity. We find seven
half-metallic magnets with 100\% spin polarization. We identify few other
compounds with high spin polarisation as "near half-metals" which could be of
potential use in applications as well. We find that the major features in the
electronic structures remain intact if a 3$d$ X$^{\prime}$ constituent is
replaced with an isoelectronic 4$d$, implying that the total number of valence
electrons can be used as a predictor of half-metallic nature in compounds from
Heusler family.
