Winter squash refers to a group of squash varieties that are harvested in the fall and have a hard outer shell. Common types include butternut, acorn, and spaghetti squash. Unlike summer squash, which is picked when immature, winter squash is allowed to mature on the vine, resulting in a sweeter flavor and denser texture. These squashes are rich in nutrients, including vitamins A and C, and are often used in soups, casseroles, and side dishes. They can be stored for several months due to their tough skin, making them a popular choice for winter meals.