Geiger counters contain a tube filled with a noble gas (usually helium, neon or argon) with a trace of halogen. When radioactive particles strike such a tube, it changes the electroconductivity of the tube. This is reported to the user by the movements of a meter needle or an audible click--sometimes both. Geiger counters usually detect only alpha and beta radiation, although there is a model that detects gamma radiation. There are more modern radiation detection devices that act like a Geiger counter but use different means to detect radiation. All of these meters report what the radiation level is at the moment without giving any indication of how long this level of radiation has been going on. They are like the speedometer of a car--they report on something that is happening at the moment it is occurring, with no memory of past events.
A dosimeter is an instrument that detects exposure to radiation over time. It is like the odometer in a car--it doesn't report what is going on at the moment, but instead reports the accumulation of what has gone on in the past. They can take many forms, but the most common type of personal dosimeter looks like a pen and is meant to be worn in a shirt pocket. They have no meter and make no sound. They are read by looking into one end of the dosimeter. The viewer sees a horizontal line that is marked with different levels of radiation and a vertical line that records the total accumulated exposure to radiation. Environmental dosimeters record the accumulated radiation at a place rather than of a person. These dosimeters look like digital readout kilowatt meters and are read the same way.
Radiation detection badges contain a small sample of a material that changes color when exposed to radiation over time. It is meant to be worn like an ID badge. They are basically dosimeters that change color to record accumulated exposure. A person can see at a glance how much radiation he has been exposed to if he is wearing one of these badges.
Environmental color changing dosimeters are called thermoluminescent detectors (TLDs) and are made of the same material as radiation detection badges and are larger, so they can be read from a distance.