From Easiest to Hardest
Hard - As the name says but do have the habit of having some aspect of "normal" (see last entry) levels making these levels something harder than their classification.
Super Hard - Would be the same as hard but tend to be loaded with specific mechanics that can end your chance of wining quickly.
Legendary - Very hard level, expect many tries, tends to have a somewhat predictable pattern.
Nightmarishly Hard - Falls often into the category of garbage leveld. These levels tend to actually be harder than Legendary and also tend to have all the worst aspects of all other level classifications.
Gentle Breeze to Category 5 Hurricane aka Normal - The only way to describe these level is... "Normal is like a box of chocolates, you never know what you are going to get". This level type can range from trivial to well beyond Nightmarishly Hard from one level to the next.