"Lines" are wrinkles.
"Cracks" are when the surface splits open, and you can see the underlying layers of the leather. These underlying layers are usually a different color, and therefore, the crack is quite noticeable.
In my bad experience with Ford leather, "lines" today become "cracks" tomorrow no matter how well you maintain the leather.
I kept the leather on my Explorer very well, cleaning and conditioning very regularly.
However, it still cracked. Sometimes when I see a similar Explorer in a parking lot, I will walk by it. I am shocked to see just how worse their leather seats look! They are not just cracked, but
are shredded! So, I guess all my hard work wasn't in vain. My seats could have been far worse than just cracked.