Can someone explain what are the differences between a YC Fellowship and a YC Core company? Is it metrics, how far they are along, founders, etc.? Are there di

Can someone explain what are the differences between a YC Fellowship and a YC Core company? Is it metrics, how far they are along, founders, etc.? Are there different things that YC looks for when evaluating a company for one vs another? What constitutes as a company being ready for YC Core? Thanks!