Many produce items are available year round now simply due to global trade. This was not always the case in the past. And just because something is in
season, doesn't guarantee it's quality either. It's always important to know your source. Not just who you're getting it from but where it's being grown when possible.
This is a good question for The Chef's Garden
The difference is simple. Both are grown in the summer but winter squash can be stored well into and through the winter in a cool dry area for months. Summer squash will not keep. Winter squash is harvested when old, summer squash is harvested when young.
Back in the days when the food supply was not global, in the days when there were no refrigerators, no canning (barely over 200 years old), no mainstream forms of preservation made possible by industrialization - winter squash was a food that was necessary to grow so that you would have food on the table when it was too cold to have a garden growing.
Winter squash is picked fully mature with a protective skin that is thick, hard, and inedible, but also protective and increases its storage life. Summer squash is picked young when the skin is tender and edible, much more perishable than winter squash.
Nutritionally, winter squash is higher in iron, riboflavin, complex carbohydrates and vitamin A, but the edible skin of summer squash is loaded with beta-carotene. Both are good for you.