Fencing, and you're about half-right.Stabs are just slashes that go straight into a person piercing them. Stabs might be quick as you say but they give openings to the user's upper body. Lunges are actually jumps towards the enemy, not sure whether they are surprise attacks but, i think that it gives the user speed and shocks the enemy as the user suddenly jumps towards him, hence giving the user a good shot at the enemy. I think lunges can be seen most in the sword fighting, with the rapiers, not sure what the sport is called again >_<...
You're not so much jumping as you are leaning forward into a step. And stabs, if done correctly, won't leave you open, because it's a quick thrust in, and a quick pull back.
Here's someone who I think describes a lunge very well, albeit in a sport way, and not in the way I teach (note, however, that the way I teach works only in the RP, and would NOT be used in real life).
You must be registered for see links
Anyway, if you understand everything, please demonstrate one.