Summer squash are harvested when tender and still immature. Summer squash grow fast, usually maturing within 2 months of planting, and continue to produce all season long. They are prolific, reliable producers, but they don't store well, so use them right away.
Most winter squashes are vine-type plants whose fruits are harvested when fully mature.
It's easy to get lost in the maze of winter squash types, so here's a quick reference of the most popular types, along with their botanical names. Pale yellow flesh. Dry orange flesh. Moist orange flesh. The latest winter squash craze.