2 types.

ok. 3 really...

1 "narrow band" cheap. nothing more than a moving gauge and a light show. only reads a very narrow range near stoich... around 14.7. it might go one full up and down, i.e. 13.7-15.7

2 wide band. expensive. this reads AFRs from ambient air at 20.8% to full rich, I've had mine reading at 10.0:1 before when I started tuning my Dakota.

3 wide band with data logging capability. for tuning, THIS is what you want. watching your AFRs on a gauge while accelerating at WOT is NOT an easy feat to do. it's much easier to use a gauge that data logs, and allows you to "play back" the run on a computer, with a graph and chart, showing you the AFRs through the RPM range, so you know where you're lean, rich, or spot on.