Zucchini or courgette is a type of squash, summer squash to be exact. Zucchini are long and round; their skin is light or dark green or yellow, and the flesh inside is white and watery and contains many small edible seeds. The stem has small prickly hairs on it, and the flesh sometimes weeps a thick colourless tasteless liquid when cut. Zucchini are quite perishable, and generally won't keep longer than a week in the refrigerator.
Zucchini can grow to enormous size if left unpicked but are ideally plucked when very young and tender, no more than 2 or 3 inches (6-7 cm) long. When small, the skin of the zucchini is still soft and so can be consumed with the rest of the fruit (yes, zucchini is a fruit, not a vegetable, though we tend to treat it as the latter). At this stage in its life cycle zucchini takes well to being quickly sauteed in a little olive oil and butter, perhaps with a little fresh basil and salt and pepper added. As the fruit grows large the skin becomes tough; at this stage it can be stuffed and baked, and the flesh and stuffing scraped out of the skin and eaten. Zucchini is good in soup or sauce, and can also be grated and used to make zucchini bread.
If you grow zucchini, don't overlook the blossoms, which are very delicious lightly sautéed in butter or stuffed and baked; see squash flower fritters for a recipe. The flowers are very delicate and won't last long, so cook them as soon as you can after picking.