Sway bars (sometimes called anti-roll bars) are an addition to suspension that give the vehicle additional stability. A sway bar is a metal rod that extends across the entire axle and joins each side of it together. Not to be confused with a dependent suspension system that also has a metal beam extending across the axle, the sway bar still allows each wheel to travel independently. These anti roll bars are designed to combat, you guessed it, body roll. Most cars come equipped with some version of these from the factory but most are more tuned for comfort and as with all suspension adjustments, when you increase comfort, performance goes down and vice versa.
Sway bars are usually attached at two pivot points between each axle. During a turn your car has a tendency to roll outwards. Meaning if you’re making a hard left turn your chassis will begin to roll towards the right and vice versa. An anti roll or sway bar is designed to combat that vehicle sway towards one side to keep the body as level as possible when cornering. When the suspension at one wheel travels vertically (this decompression would occur on the inside wheel) in a turn the sway bar transfers that movement to the other wheel. This transfer of energy from the wheel under stress to the other side of the suspension essentially creates a force to resist that body roll via the anti roll or sway bar. This reduces vehicle sway, hence the name anti sway bar. Most modern vehicles come with this suspension system standard and there is a plethora of aftermarket options available for most vehicles and the install is pretty straight forward and those vehicles that don’t come with stock bars usually have mounting points for them in the suspension components.
Some higher end and luxury cars eliminate the need for anti rolls bars with active anti roll technology. Which is pretty much just a fancy computer system that detects body roll is occurring and adjust the fluid pressure in the appropriate shock or strut assembly, simulating the same resisting force the anti roll bar would be providing.
As with modifying any part of your suspension system there is a kind of give and take relationship. Increasing the performance in one aspect of performance could be detrimental to another. This is a constant trial and error process to dial in the suspension for the drivers needs.
Strut braces serve the same purpose as anti roll bars just in a different way. As your suspension travels to adjust to the road and corners, it undulates all of it’s components in sync to deliver the idea ride. Sometimes you want to constrain some of that motion to squeeze more performance out of your suspension. Strut braces connect your suspension pillars that are exposed in your engine bay. This direct connection via the strut brace produces the same stiffening effect the anti roll bars do.