Courgettes are young marrows, but courgette varieties have been bred to be picked young, when they have a sweet flesh and thin skin. If courgettes are left to grow large to become marrows, their flavour becomes less sweet, the seeds within become much bigger, and the flesh thickens. Whereas courgettes can be eaten whole either fresh or lightly steamed, marrows are usually cooked by being hollowed out (cut in half with their seeds removed) and stuffed, before baking in the oven.
Courgette and marrow types differ in their growing habit. Courgettes usually grow in a bush shape, with their fruit centred in the middle of the plant protected by the leaves above and around them. Whilst courgettes can be grown in pots, they are big plants with a spread of about 1 metre, and they require plenty of water whilst setting fruit. There are dwarf varieties available, but in general, courgettes are best planted in open ground where they have plenty of soil and space to grow. Marrow varieties need even more space, as the plants grow bigger and with a trailing habit (more like squash).
Shops often sell one type of courgette, the straight green variety. A gardener can grow a variety of different shapes and colours. Whilst this adds interest and colour to a garden, I have not found the taste of courgettes to be greatly different between varieties, or indeed those sold in the shops. What I have experienced is how a few mature courgette plants can produce a phenomenal quantity of fruit. Once the plants are established in midsummer, and provided they are well watered and the fruit is picked young, each plant can produce several courgettes a week. A glut of courgettes is a common problem as courgettes do not store well, but they can be chopped, blanched, and frozen.
Like squash, courgettes are frost sensitive plants and they should only be planted outside when all risk of frost has passed. It is common to start seeds indoors or under protection in a greenhouse or polytunnel. Young seedlings with tender stems and leaves are vulnerable to being eaten by slugs and snails. To avoid this, allow the plants to grow bigger before risking planting out, as bigger plants have thicker leaves and stems and less likely to eaten. Plant courgettes, and marrows, in really rich soil, and ensure that this is kept moist, especially when the plants are setting flowers. Picking courgettes regularly when young will encourage the plant to set more flowers and extend the harvest season.