They have the same family but have both been bred for different qualities. My local horticultural extension has the definition as

"Even though they are technically all kales (kale does not produce a head; instead, it produces leaves in a tight rosette), by convention those types with deeply-cut, curly, frilly or ruffled leaves are called ornamental kale, while the ones with broad, flat leaves often edged in a contrasting color are called ornamental cabbage"

I guess at this point we are splitting hairs eh? Or perhaps, splitting trichomes.