Scientifically they are all squash. The names of the main species which are cultivated are:

Cucurbita. pepo - zuchinni, acorn squash, Jack O Lantern type (orange) pumpkins, delicata, etc.

Cucurbita maxima - Big Max, Atlantic Giant, Hubbard, Turk's Turban, Banana squash, Buttercup, Green Hokkaido, Rouge d'Etampes, Kuri Blue, etc.

Cucurbita mixta - most Cushaws, Tennessee Sweet Potato

Cucurbita moschata - butternut, Tahitian Melon squash, Golden Cushaw, Cheese pumpkins, Musuee de Provence, Neck Pumpkin, Seminole, Tromboncino (which makes a good summer squash)and, of course, my favorite = Warsaw Buff Pie Pumpkin

These families pretty much don't cross pollinate. But within the families cross pollination is VERY easy.

"Pumpkin" is really just a term, mainly referring to shape, I believe. But some squash, which are called pumpkins, are not round. So the distinction is both artificial and unclear.

Hope this helps!

George
Tahlequah, OK

What makes it confusing perhaps is that the Blue Squashes are very popular in Australia and New Zealand. There they call the same thing 'pumpkin' that is called 'squash' in the USA.

For example:
Queensland Blue Pumpkin in Australia
Queensland Blue Squash in the USA

George already said that botanically they are all the same (within their families) and whether they are called pumpkin or squash is just local convention.

"Squash" is part of an Algonquian Indian word. "Pumpkin" is an English word, originally derived from the French "pompion," meaning melon (and originally deriving from the word "pepo" in Curcubita pepo). "Pepo" or "Pepon" means "melon" in Greek. when the first settlers in N. America encountered squash/pumpkins, they had no word for them, so they used a variant (actually just the last chunk) of the local Algonquian word. The plants eventually reached England, and the English used the nearest word they had to describe them. Nowadays in the US we generally call the orange more-or-less round ones "pumpkins" and the rest "squash" but the English tend to prefer "pumpkin" for all the winter squash, and "courgette" for what we in the US call summer squash. But botanically there's no basis for the difference as what we call "pumpkins" can be from three different species.