Analysis of information sources in references of the Wikipedia article "Marrow (vegetable)" in English language version.
Courgettes are commonly described as marrows harvested young. However, there are some slight horticultural differences between courgettes and marrows. Courgettes [Courgette plants] tend to be bushy and [have] thin-skinned [fruit], whereas marrows [marrow plants] tend to trailing and have [fruits with] a thicker skin.
Then there are the zucchini a sort of green squash.
Pare and slice in medium slices six zucchini and fry for a few minutes in olive oil.
The general difference between Marrows and Courgettes/Zuchini is; Marrow plants tend to trail out and the fruit skin is quite thick, whereas Courgettes grow as a bush and the skins are quite thin.