Bars are hand rails used by the player to brace themselves while they play ITG. Rarely, players will choose to play without holding the bar (referred to as "no bar"), but most people hold on to the bar.
Dance Dance Revolution (DDR) arcade bars are completely symmetrical and narrow, making them ideal for smaller players. Taller players may have trouble holding these for extended periods of time. They are, however, extremely durable. They are welded to a steel base but also have another platform that attaches to them further up the bar. These extra attachment points make DDR bars very sturdy in comparison to ITG/PIU bars.
Pump It Up (PIU) arcade bars are asymmetrical. The bar is an odd shape that swoops in, leaving a large bulbous end that sticks further out from one side. For taller players, the extra width is nice, but you have more extra-er width on one side, which can lead to a bit of shoulder pain on one side. Older PIU bars bolt down to a base while newer ones were welded, but they do not have the additional attachment points DDR bars have so they often crack and need re-welding at some point in their life if used heavily.
In The Groove (ITG) arcade machines, being partially-developed by Andamiro who also developed Pump It Up, share a very similar bar design. They are asymmetrical like PIU bars but there is no inner swoop. Instead, the bar just extends out from one side, much like your rotator cuff will extend out on one side from using one of these bars. Because ITG machines are somewhat rare, these bars are rare and any bars that break are usually repaired. Like PIU bars, they crack at the base rather easily.
There are a few different variants of ITG bars.
Pop'n Stage was an obscure dance game based on the more popular Pop'n Music rhythm game. It was notable for having large triangular bars (sometimes referred to as "Dorito Bars").
StepManiaX (SMX) bars are symmetrical, like DDR bars, but they are wider so they seem to suit taller players. This bar is quite sturdy, attaching to the base using a very large bolt (like the ones that bolt down streel lamps), so there is less pressure put on specific welds like older arcade bars.
There is a prototype SMX bar that has a welded flat flange (instead of bolting down with the huge bolts), but Kyle only made a handful of those which he sold to various players for DIY projects.
There is also a first-party adjustable SMX bar. These are pretty wobbly and people usually have to modify them slightly to make them more sturdy.
The PVC mod attempts to make the bar wider (and sometimes closer) and make the hand position easier on the wrists. This is done by buying a PVC pipe, and either attaching it to the bar with clamps, or slicing the pipe lengthwise and hose clamping it on top of the existing bar.