Class, which refers to the technical difficulty of a route, ranges from Class 1 (walking on a well-maintained trail) to Class 6 (using climbing equipment and engineering skills to ascend a cliff).

Class 1: No hands.	Class-1 is walking down a trail. The trail might be steep and rocky, but you could do it with your hands in your pockets while whistling a happy song.
Class 2: Occasional hands, no fall.	Class-2 is boulder hopping. The route might be a rocky wash or a rocky ridge, and you would occasionally use your hands to steady yourself while whistling a winded, but happy song.
Class 3: Hands, short fall.	Class-3 is easy climbing. The route might be steep terrain or rocky outcrops where you need your hands. There would be lots of large handholds, and while Class-3 is easy and you wouldn't fall, a fall none-the-less would be short and result in injury but not death. You wouldn't do it with your hands in your pockets, but you probably wouldn't want a rope either, and you still might whistle a happy song (at least after you got over the outcrop).
Class 4: Hands, long fall, maybe ropes.	Class-4 is easy climbing over steep, exposed terrain where you need your hands. There would be many large handholds, and while you probably wouldn't fall on Class-4, a fall none-the-less would be long and would result in great injury or death. You wouldn't do it with your hands in your pockets, you might want a rope (especially for the down-climb), and you might be a bit too puckered up to whistle a happy song.
Class 5: Hands, long fall, ropes (climb using hands and feet on the rock).	Class-5 is serious rock climbing with ropes, but you don't hang on the equipment or use it to pull yourself up the rock. There could be handholds, maybe not, but they wouldn't always be where you needed them most. You probably would fall on Class-5, and the fall would be long and result in sudden death unless your belayer saved your life. You wouldn't do it with your hands in your pockets, you would want a rope, and you would be a bit too puckered up to whistle a happy song. Class-5 climbing often is referred to as "free climbing," and climbing 5th-class routes without a rope often is referred to as "3rd-classing the route" (climbing as if it were 3rd class and you didn't need a rope). Class-5 is graded according to the technical difficulty: Under the American system (Yosemite Decimal System): Class-5.1 is easy 5th-class Class-5.7 is moderate 5th-class Class-5.10 is hard 5th-class Class-5.14 is impossible 5th-class (at least for me).
Class 6: Hands, long fall, ropes, climb using equipment (not hands and feet).	Class-6 is serious rock climbing with ropes where you hang on your equipment and use it to pull yourself up the route. Class-6 climbing often is referred to as "aid climbing" because you use your equipment to "aid" in your ascent. There are not necessarily any handholds. You might fall on Class-6, and the fall could be long and result in sudden death unless your belayer saved your life. You wouldn't do it with your hands in your pockets, you would want a rope, and you might be so puckered up that you wouldn't even be thinking a happy song. Class-6 is graded according to the technical difficulty of setting the equipment and the length of the potential falls (using "A" as an abbreviation for "Aid"): A1 is easy aid climbing where placement would be easy and secure; no falls A2 is moderate aid climbing where placement would be difficult but generally secure; short fall A3 is hard aid climbing where placement would be difficult and insecure; moderate length falls A4 is nearly impossible aid climbing with potentially long falls.

In the U.S., grades refer to the length of time that it normally takes to complete a route. Grades range from I (a short walk) to VI (three or more days on the route). Grade is not directly related to technical difficulty, as you can do an expedition-length route on a well-maintained trail or an exceptionally difficult aid-climbing route under a boulder, but harder routes tend to take longer and tend to have higher grades. Keep in mind that a beginner might take all day on a Grade-II route, while a Grade-VI climber might jog a Grade-IV before lunch.

Grade	Brief Description
I.	A brief outing, an hour or so.
II.	A short route, less than 4 hours.
III.	A moderate route, lasting most of the day; bring a lunch.
IV.	A long route, lasting all day; bring a lunch and extra food.
V.	A long route, lasting two days; bring several meals.
VI.	Expedition-length routes lasting three or more days.