The difference between too much trail and too little has been experienced by all of us. Think about going around a really sharp turn: take it too tight, and too slow, and you'll dab to the inside. You've oversteered.
Now think about the last time you over shot the same turn: you've
understeered and ended up in the woods. Why does this happen? A good
part of steering performance rests in the amount of trail. Too much
trail and you'll oversteer (some feel this as wheel flop or dive) at slow speeds and understeer (overshoot the corner) at high speeds. Too little trail and you'll understeer at slow speeds and oversteer (feels twitchy) at high speeds.