A nuc could be described as a very small hive, but it is really the nucleus of a hive: the core if you will. Often a nuc consists of five frames of honey and brood as well as the bees that would cover those frames. A nuc is not a full hive that will over-winter, but given sufficuent time, it will grow into such a hive. Consider a nuc an intermediate step between a package and a fully developed hive. Nucs serve other purposes as well, I understand, such as swarm prevention and providing resources for emergency situations, bit others are far more knowledgable about these things than I.