  Almost a decade has passed since the serendipitous discovery of the
iron-based high temperature superconductors (FeSCs) in 2008. The question of
how much similarity the FeSCs have with the copper oxide high temperature
superconductors emerged since the initial discovery of long-range
antiferromagnetism in the FeSCs in proximity to superconductivity. Despite the
great resemblance in their phase diagrams, there exist important disparities
between FeSCs and cuprates that need to be considered in order to paint a full
picture of these two families of high temperature superconductors. One of the
key differences lies in the multi-orbital multi-band nature of FeSCs, in
contrast to the effective single-band model for cuprates. Due to the complexity
of multi-orbital band structures, the orbital degree of freedom is often
neglected in formulating the theoretical models for FeSCs. On the experimental
side, systematic studies of the orbital related phenomena in FeSCs have been
largely lacking. In this review, we summarize angle-resolved photoemission
spectroscopy (ARPES) measurements across various FeSC families in literature,
focusing on the systematic trend of orbital dependent electron correlations and
the role of different Fe 3d orbitals in driving the nematic transition, the
spin-density-wave transition, and implications for superconductivity.
