In a simplified view, the hippocampus receives its major input from layers 2 and 3 of the entorhinal cortex (EC), and outputs to layer 5 of the entorhinal cortex. The dominant anatomical features of the hippocampus are consistent with the notion that inputs from the EC are processed at successive levels, from the EC layer 2 spiny stellate cells to dentate gyrus (DG) granule cells, to recurrently connected CA3 pyramidal neurons, onto CA1 and subicular pyramidal cells (along with a direct projection from EC layer 3), and fed back to the EC layer 5. Understanding the nature of information processing at each stage and the functions of the individual input/outputs is a key goal of our lab.
For additional details, I recommend the excellent chapter by Johnston and Amaral in Gordon Shepherd’s Synaptic Organization of the Brain, and the Scholarpedia webpage curated by György Buzsáki.