The biggest difference between a regular cake and a sponge cake is the fat content. Sponge cakes generally contain very little or no added fat.  Also, most sponge cakes use no baking powder or baking soda. Instead the well beaten eggs or egg white work to give the cake airiness and lift. The foaminess of the beaten eggs results in a spongey texture which is excellent to absorb the syrup and become moist.