I was disappointed in the US to find that the excitingly named rutabaga turned out to be a swede.
Turnips are a variety of Brassica rapa (as is pok choi and of course, rape – which Americans can't say, so have to call it something else– fact fans), swedes are a variety of B. napus, a cross of B. rapa and B. oleracea – basically a cabbage did it with the turnip and they had a big baby.
Kohlrabi is a german turnip, that's pure cabbage.
Broccoli, cauliflower, cabbage, sprouts kale, etc. are all the same species (B. oleracea).