A squash is a fruit of the gourd family which has been a food source in the Americas for thousands of years; squash, along with corn and beans, were first cultivated by First Nations people, who refer to this trinity as the Three Sisters. Today there are many types of squash which vary widely in size, shape, and colour, but are usually subdivided most broadly into summer squash (also known as marrow) and winter squash.
Summer squash, as their name implies, ripen in the hot summer months. If picked while young, they have thin edible skins and soft seeds, though if they are left to grow after the flower falls off, they become large and thick skinned. (Anyone who's missed a zucchini when picking squash off a vine will know what huge monsters they can morph in to, left to their own devices.) The flesh of the summer squash has a high water content and mild flavour and is best cooked quickly in as little moisture as possible; sauteing is my preferred method. Fine upstanding examples of summer squash brigade are the long and slender zucchini or courgette, which can be dark green, light green, or yellow skinned; the larger yellow skinned and sweet crookneck squash; and the small flat green or yellow pattypan or scallop squash, which has a mild nutty flavour. All should be bought when small in size, and should be firm to the touch with a glossy skin. Keep in plastic bags in the refrigerator, and use as soon as possible.
Winter squash come into their own in the autumn months. These babies tend to be "of a generous size", as the Joy of Cooking delicately puts it, and come in attractive shapes and colours. The skin is hard and thick, hence inedible, and the seeds too are hard and must be cooked before being eaten; see pepita for directions. The flesh of the winter squash is orange and firm, and hence needs longer cooking times than its summer cousin; winter squash is very good steamed, baked, stuffed, and/or pureed in soups. They can be tricky to cut up, so see How to peel and dice an acorn squash for pointers. Venerable members of this family are the Halloween favourite, pumpkin; acorn squash, which is shaped, as you might suspect, like a deeply ridged acorn and has slightly sweet but rather bland flesh; buttercup or kabocha, with dry flesh, though sweet; butternut squash, a long tan number with a knob on one end and sweet, dry flesh; hubbard squash, which can be huge (up to 20 lbs); and the weird spaghetti squash, whose flesh cooks up into noodle like fibers. Choose a squash which is heavy for its size and has a thick, hard, unblemished shell. A whole squash will keep for up to a month in a cool, dark area.
If you grow squash, be sure not to miss out on the blossoms, considered a delicacy. Both summer and winter varieties have them, and they should be used as soon as possible after picking. Wash them only if dusty, and be sure to pick off any insects. You can sprinkle them raw on mesclun for a classy looking bistro salad, or coat them with a light batter and fry them. Or stuff them with soft cheese and bake till the cheese is hot. Or see courgette flower fritters.
Squash is also a name, most heard in my experience in British colonial contexts, for a fruit drink made from concentrate. Orange squash is most common, but companies like the Indian Rasna (www.rasnainternational.com) offer also mango, pineapple, and lemon squashes, as well as a variety of cordials and other fruit drinks. (I confess I cannot discern here or elsewhere what the difference is between squash, cordial, and fruit drink.) In any case, their website informs me that their squashes are sold in Attractive Frosted Bottles. Further, these Premium Squashes contain Real Fruit Pulp, and each 500 ml Bottle makes 15 glasses of Real Fruit Squash. Sorry, ariels.