You know how your car stereo can tune to different radio stations because they broadcast on different frequencies? Police radar guns also transmit on different frequency ranges, or different bands.
X band (around 10.5 GHz) is the oldest technology and least common. With its huge antennas, X band has been phased out of most of the country, except for Ohio, New Jersey, and a few rural places across the country. Since you’ll get false alerts from some shopping center door openers, most people now shut off X band detection altogether.
K band is newer and operates around 24 GHz. Police use K band throughout most of the US. Unfortunately there’s a lot of other sources of K band besides police radar including speed signs, automatic door openers, and many new vehicles with blind spot monitor (BSM) radar and smart cruise control. For that reason, K band false alerts are common and you’ll want a modern detector with good false alert filtering.
Ka is the newest and most common band and and you’ll find it on 33.8, 34.7, and 35.5 GHz. It has the smallest antennas (because it uses the highest frequencies). Ka falses are also pretty rare so if your detector alerts to Ka, pay attention.