Most organized rides give you a heads up on the level of difficulty, the below descriptions should give you a pretty good idea of what these levels actually mean! I have no idea where I got this information, but it has proved to be accurate through my own experience.
Easy, short ride, mostly level terrain, few climbs or hills, good footing. Usually follows roads or wide, well defined trails most of the way. 1 – 3 hours long.
Generally good footing. Some climbing and downhill, but not steep or difficult. Can go cross country on mostly level ground.
Footing may be difficult in places, or rocky. May have steep climbs and downhills, and narrow or precipitous trails. Requires confidence in your horse. 4 – 6 hours including lunch break.
Steep, rocky, may require horses to step up and down large rocks or negotiate narrow trails with steep drop-offs. Requires total confidence in your horse. 5 – 7 hours including lunch break.