A difficulty scale, from category 4 (the easiest) to "hors catégorie" (the hardest).
The category combines the length of the climb and its average gradient, sometimes adjusted for where it falls in the stage (a climb near the finish "counts" for more).
In increasing order of difficulty: cat. 4 (a small rise), cat. 3, cat. 2, cat. 1, then hors catégorie (HC) for the giants such as the Galibier or the Tourmalet. The higher the category, the more points the summit is worth in the mountains classification.
See also: The mountains classification