Because the radiation in our surroundings is odorless, tasteless, touchless, etc, we need instruments to measure its presence and effects. Therefore we need to understand what dosimeters and geiger counters are, what's the difference between them, and the numbers they're telling us.
Dosimeters measure an individual's or an object's exposure to radiated energy. Older types of dosimeters, Film badge dosimeter or Quartz fiber dosimeter, are being superceded by modern Electronic Personal Dosimeter's. The latter are personal electronics devices with an LCD screen etc. The idea is to measure the dose received over a time period.
A Geiger–Müller counter, also called a Geiger counter, is a type of particle detector that measures ionizing radiation. They nuclear radiation — alpha particles, beta particles, and gamma rays — by the ionization produced in a low-pressure gas in a Geiger–Müller tube.
There are two kinds of measurements of radiation: The count of radioactive events, and the dosage.
Geiger counters usually have an option to make audible clicks for each ionization event. They also have a display that's either analog or digital showing the measurements.
The output pulse from a Geiger-Müller tube is always the same magnitude
regardless of the energy of the incident radiation, the tube cannot
differentiate between radiation types.
Since alpha waves are so weak, they don't travel very far even in air, and the G-M tube must be pretty much unprotected in order to detect them. Detecting beta waves is easier, and the G-M tube can be behind a thin wall. G-M tubes are fragile and should be kept protected.
Because a G-M tube cannot distinguish the energy of the radiation it cannot tell you whether this is cesium-137 or anything else.
Radiation at 1 CPS (count per second) is equivalent to 1 Bq. But the useful measure is the "gray" or sievert, because that is the measure of the effect on humans. Both are measured in Joules per Kilogram of mass being affected by radiation. Joules is energy - which is an entirely different measure than Counts.
Radioactivity is often expressed in becquerels per unit of volume or
weight, to express how much radioactive material is contained in a
sample. Hence, we'll see measurements like Bq/Liter (a unit of volume). Again, this is a number of radiation events per liter of volume, which doesn't tell us the radioactive energy emitted from the material.
An NIH paper, Radiation Dosimetry, makes it seem that Dosimeters measure the energy of the radiation being detected. That's what is desired for the common use case of a dosimeter - for example, a radiologist runs X-Ray machines all day, and needs to track his/her exposure to radiation over a given period. In order to minimize the risk of getting cancer, workers who use radioactive materials in their job have to track how much radioactive energy they've been exposed to.