The terms "strut bar", "stress bar", and "strut brace" are all generally refer to the same kind of device, a bar mounted between the front strut towers.  The purpose of this bar is to reduce the flex that these towers experience during hard cornering. Reducing the flex will keep the wheels in position and in the desired position on the road, allowing the suspension to work as engineered without the added factor of chassis flex changing camber and caster settings.

This will help to improve traction on the turns, increasing turn-in response, and help the suspension work effectively. However, unless you choose carefully you may end up with a bar that looks great but does not meet your legitimate performance objectives.

Many of these bars look sturdy, shiny, and just plain cool. However, looks have little to do with performance. To reduce the flex between struts the stress bar setup should be as rigid as possible. The bar should be designed to take horizontal pressure during a turn without flexing or losing rigidity. The bar should not bend, flex, or pivot at the point it is attached to the strut cap otherwise the towers can move freely as they did before:

By solidifying and isolating the struts in their original configuration, it allows the suspension components (stock or modified) to retain their geometry and to function as designed.  Typically, steering  will seem quicker and more responsive, but in reality, it is the suspension doing its job properly.  Benefits are not limited to high performance driving.   Most people find a strut bar improves low speed turn-in response as well.   A final advantage is the structural longevity of the vehicle; by giving everything extra support, the chassis will retain that "like new" stiffness for a much longer period of time.  In particular, convertible model owners will see noticeable decreases in cowl shake.

From driver school participants and autocrossers to the typical enthusiast, the strut bar has become a widely accepted solution to the annoying problems of chassis flex in modern production cars and professional race cars.

Want to see the serious math on how a good strut bar works?  [ CLICK HERE ]