The main differences between Summer and Winter squash are skin thickness, seed size, flesh density and storage longevity.
Its skin is soft and fruit is consumed when young. Skin is usually edible. The fruit usually spoils very quickly. Its flesh is soft and delicate. Seeds are small & soft, easy to scoop. Example: zucchini.
Its skin is hard and fruit is consumed when matured. Its flesh is dense, seeds are hard and three times bigger than in Summer Squashes. Winter squashes are easy to store and can last for a long time (3 to 6 months). Examples: kabocha.