Ski Trails are graded for difficulty.
In Europe ski trail grading goes from blue, easiest, to red, hard, to black, hardest.
France also includes green, really easy.
In the USA and Canada ski trail grading goes from green to blue to black, with addition of shaped symbols (circles, squares and diamonds). With the hardest being double black diamonds and very occasionally triple black diamonds.
However in all resorts the grading systems are relative. For instance an icy red in poor condition can be much harder and dangerous than a freshly groom black piste in good condition. Additionally difficulty, can and does vary from resort to resort and especially from country to country. While the steepness of the piste, is to a large extent, dependant on the steepness of the mountains themselves.