Class 2 trails may require the use of hands for support. A couple of rocks but still no climbing. Class 3 trails contain some rocks. Inexperienced climbers may wish to use a rope, but it's still fairly easy. Class 4 has more difficult rocks.
Many climbers choose to use a rope for safety.
Usually, natural protection is easy to find. Class 5 is free climbing. A rope and protection are required. Class 5 is subdivided by the Yosemite Decimal System into fifteen groups from 5.0 to 5.14. Most beginners start in this range.