Squash is an all-encompassing term for the fruits of the gourd family of plants. This includes pumpkin, butternut squash, and acorn squash, all of which are Paleo.
Carbs vs. Nutrients
Despite the fact that squash tends to be fairly high in carbohydrates, living a Paleo lifestyle is all about eating plenty of nutrient-dense foods and squash have nutrients to spare. They come loaded with various B vitamins, as well as vitamins A and C, and minerals such as iron, calcium, magnesium, phosphorous, potassium, and zinc.
Squash as Smart Fuel
Squash are such an excellent source of nutrients, that Mark Sisson labeled thre of them as a smart fuel here, here and here.