The key difference between regular Ubuntu and Ubuntu Core is the underlying architecture of the system. Traditional Linux distributions rely mostly on traditional package systems—
deb, in Ubuntu’s case—while Ubuntu Core relies almost entirely on Canonical’s relatively new
Ubuntu Core also gets a full 10 years of support from Canonical rather than the five years traditional Ubuntu LTS releases get. But it’s a bit more difficult to get started with, since you need an Ubuntu SSO account to even log in to a new Ubuntu Core installation in the first place.