An open trail can be english or western as far as I know. Although, they could be using the word "open" to indicate no diferentiation between child, junior or adult.
In any case,t eh english version isn't much different then the Western version but it really depends on who puts the "course" together. In the english trail classes I have seen, gaits aren't set and the placings are based on time to complete the course rather than "skill" so riders can set their gaits based on their horse's ability. Other classes will judge on skill and levelheadedness (is that a word? :) ) of the horse.
In other words :), it can be anything but as wares indicated, trotting poles, maybe a small crossrail, walking over "things" like tarps or some such, weaving around cones, being required to pick something up in one location, carry it to another and place it again, making the horse stand for a certain period of time inside a box, possibly dismounting and remounting (though I notice this particular obstacle isn't done as often anymore) etc.
If your horse has a good head ( as in proverbial common sense :) ) and isn't the spooky type, a trail class is a lot of fun.