Tripwires are some of the oldest traps known to man and in the Dragon Age they are used a lot. You can find them in many of the dungeons around Ferelden. Simply pulling the string will set off an effect, whether it is a ballista firing or a barrel exploding or just plain damage to one of the companions.
It's possible to detect these traps with enough awareness and to disarm them with enough skill as rogue's deft hands. Disarming them is a rewarding business, as not only will your party be safe but you will also gain valuable experience (usually between 5 and 50 XP). The more XP you get the higher your skill has to be to disarm them.
[PC Version 1.02] The tripwire shown in the picture (found in the southern cave in The Dead Trenches, just after the Ancient Darkspawn) cannot be disarmed. It will also reset itself every time you come back to it, as long as you leave its immediate vicinity. It appears to have no effect beyond playing a screeching sound.