Both are made from whipped eggs and caramel syrup. Why nougat is viscous and meringue is soft and can become dry and crunchy?
The basic difference is consistency: meringue tends to be lighter, airier, and drier, while nougat is more chewy.
This is achieved by different ratios of sugar to egg whites. Nougat has significantly more sugar in proportion to egg whites. (Compare this nougat recipe, ~4 cups sugar/honey to four egg whites, and this meringue recipe, 1/4 cup sugar to four egg whites.)