Hippocampus is a part of the system called limbic system and received highly processed information from all of sensory systems.  Hippocampus is the first place where neuronal plasticity was observed in vivo by Bliss and Lomo.  The neuronal plasticity is called long-term potentiation (LTP) that is the activity-dependent enhancement of neuronal transmission.  Since the discovery of LTP, hippocampus attract a major attention of researchers in the field of learning and memory. 
    The current knowledge on hippocampal roles in memory is its involvement of consolidation of memory, i.e., processing of short-term declarative memory into long-term one.  Bilateral lobectomy of hippocampus to treat severe epileptic seizure is reported in human.  After the lobectomy (removal) of medial temporal lobe, the patient (H.M.) suffers severe anterograde amnesia with normal memories of his child days.  As long as repeating the fact, he can remember it (short-term memory), but when his attention is distracted he immediately lose his memory.  He even didn't remember what he was doing.  He reads same magazine repeatedly without noticing he had read it.  He brushes teeth but he can not remember where his toothbrush was.  Interestingly, H.M. shows normal ability to learn skills, movement of body (non-declarative memories).

    The symptoms observed in H.M. tell us important roles of hippocampus and features of memory:
1) Short-term memory doesn't requires hippocampus,
2) Rehearsal (repeating) and attention are required for shor-term memory,
3) Recollection of past memory such as childhood memory doesn't require hippocampus,
4) Childhood memory is not stored in hipocampus
5) Declarative memories requires hippocampus
6) Non-declarative memories are dealt with other  part of the brain