They’re part of the brassica family (like cabbage, kale, and broccoli) and are now in peak season (typically late autumn through winter). They grow on tall stalks, with lots of small green “mini cabbages” attached along the stem.
 
A few quick notes that might help from The Little Farm Shed:
Storage: Keep them in a cool, dark place (like the fridge) and they’ll last over a week.
Popular prep ideas:
Roasted with olive oil, salt, and maybe some honey or balsamic glaze.
Shredded raw into a winter slaw with apple and nuts.
Pan-fried with bacon or chestnuts (a classic!).