Grouping and life of crops

Brassicas - anything cabbage/kale related including sprouts
curcurbits - anything sprawly with fruits that need lots of water to swell, cucumbers/squash/pumpkin
root vegetables - anything that grows below ground and you pull up and eat - carrots/parsnips/celeriac etc
aliums - onions/garlic/shallots/spring onions etc
anything else - spinach/lettuce/radish....

basically, it's all about rotating stuff grown so that you don't encourage diseases to take hold in your soil, and to not deplete the soil by growing the same thing in the same place. Also by rotating, you're giving yourself the chance to provide good growing conditions for next years plants this year, by thinking ahead ot what they're going to need.
You can use some of your potatoes for next years crop if you like, and save seed from your onions if you let some flower. You can also save bean seeds, pumpkin/squash/courgette/sweetcorn/ seeds, garlic bulbs.
There's not much you can't save the seed from, but you could end up with some interesting crosses.
Plants that come up again and again are generally, artichoke and asparagus, strawberries [fruit bushes and trees], but asparagus are the only ones you leave alone for 20 years, the other ones you either split, or grow new plants from runners.

I think the brassica group is quite confusing, because it includes some leafy veg and some veg that grow below ground;

cabbage
cauliflower
kale
broccoli
calabrese
sprouts
pak choi
mustard greens
kohl rabi
turnips
swede

Thanks.

So far I'm looking at growing (some plants, some from seed):

Tomatos (plants)
Cucumber (plants)
Sweetcorn (plants)- curcurbit?
Carrots (seed)
Parsnips (seed)
Spring Onions (seed)

That'll start me off in the veg world for this year.