NPSH = Nett Positive Suction Head - This is the head that is on the suction side of the pump. Every pump has a requirement that the manufacturer will specify this on its datasheets. It is different for every pump and system.
NPSHA = Available - This is what is actually available on the system. This must always be greater than that required by the pump to prevent cavitation.
NPSHR = Required - This is specified by the pump manufacturer once they have the duty details.