Most of the time, these labels are focused on CARE levels, rather than the typical disposition of the snake...though that comes into play as well.

'Beginner' snakes are for those with little to No experience caring for a snake of any kind. Snakes in this category should have simple needs, a wide acceptance range for temperatures, and stay small to 'reduce the amount of space they take up'. Of course, keep in mind that no snake should be a 'baby pet', and any gotten for kids under 12 should be the full responsibility of the parent while the child watches and learns.
These, to my mind, would include Gaters, Ribbons, Water snakes, and most of the small colubrids like kings, rats, corns and milks.

'Intermediate' snakes are for people with some experience, who know the special differences and rules about snake keeping (like escape-proof enclosures and feeding schedules). Snakes here would still be 'easy' to care for but would now need things a bit more stable and would require a higher output of money as well.
Ball Pythons, to me, are here along with Boas (RTB, Rosy, Sand) and Hognose snakes and the like. (It's early, complete lists aren't coming to me)

'Expert' snakes are the ones for people who have learned well about snake keeping, have done it for a while and are ready for a challenge. In one way or another, these snakes need advanced care...be it a large amount of space, special diets, or special handling/permits.
The Giants (Burms, Retics and Condas) would be here along with GTPs and all Hots.

There's my take on it, at least.