They differ from baseline humans physiologically with the Mazarin-Rashoud Node set between their frontal lobes. For most novas, that is the extend of variance; however, the effects of specific quantum powers and/or Taint mutations can cause an individual nova's physiology to differ greatly from the human norm.
Novas gain powers in their moment of Eruption. This process starts in a moment of immense shock, and for a few short moments allows the Nova to gain the full power of Quantum. However, the human consciousness cannot take it for long, so subconsciously it creates powers that are easier for the mind to understand. Usually these powers and their effects are chosen subconsciously in the short moments of Eruption.
The Galatea Disaster was the source of the eruptions on March 23rd of 1998, when the focused radiation from that space station's explosion blanketed the Earth. The first (public) appearance of a nova was Randel Portman, the fireman from the covers of the Aberrant and Aberrant Players Guide books, whose eruption occurred later that day.
The first ProtoNova was Divis Mal, who created the Galatea Disaster to quicken the evolution of humanity.