It's funny how we decide to give different vegetables different names. How come apples of all shapes and sizes are apples - but we have cucumbers, green squash and zucchinis?
First off, all squash are of course related, including yellow squash, butternut squash, etc. We're just talking about the green ones here. The main green squash that you see are the cucumber and zucchini. All other green squash are then lumped into a descriptive category of ... green squash.
I love cucumbers! My favorite recipe involves cutting them into half moon slices and soaking them in soy sauce, vinegar, etc. Not only that, but they have only 1.8g carbs per 1/2 cup. Be sure to include cukes on your salads, as a snack, whenever you can.
Zucchini are great for you too. They have 3.3g per 1/2 cup, so slightly more, but still really low carb and full of nutrition. Zucchini are great when they're grilled up and served with steak or grilled chicken or seafood.
Really, green squash usually refers to zucchini, but it can refer to other squash that happen to be green in color. Depending on the variety, they would have around 3g per 1/2 cup.
Lisa Shea's Library of Low Carb Books