Radiation detector is a term used often synonymously with Geiger counter. However, while the most widely used radiation detectors and monitors do, in fact, utilize Geiger-Mueller (GM) tubes, others may use very different technologies. 
GM tubes are classified as gas-filled detectors. All such detectors consist of a volume of gas between two electrodes. Such devices indicate the number of interactions occurring in the detector and are thus called counters.

Radiation detectors that utilize the interaction of ionizing radiation to produce UV and/or visible light are called scintillation detectors. Scintillation detectors yield information about the energy distribution of the incident radiation and can thus be utilized in spectrometers. The most common form is the NaI (sodium iodide) scintillation detector. 
Finally, radiation detectors that use especially pure crystals of silicon, germanium or other materials which act as diodes are called semiconductor detectors. Certain of these devices, which indicate the net amount of energy deposited in the detector, can be utilized in dosimeters. Others can be configured for spectroscopy. 
Each type of radiation detectors has its advantages and disadvantages. Those incorporating GM tubes, such as the R500 Radiation scanner have become the most commonly used due to ease of handling and low cost. For most purposes they are sufficiently accurate and reliable. For applications where high efficiency for gamma radiation is sought, scintillation devices are best suited. Scintillation detectors are commonly used in medical applications such as digital radiography, fluoroscopy or CT scans. Most scintillation detectors will not detect alpha or beta radiation. 
Geiger tubes are rugged and relatively inexpensive to manufacture. Spectrometer devices are often both expensive and fragile, and are used mainly in high end medical applications, or for security applications. 
When scintillation detectors or any other gamma-only detectors, are used for safety purposes, it is important to understand their strengths and weaknesses. There are situations in which alpha and beta radiation can be present in high concentrations, with little incident gamma radiation to indicate a serious health concern. 
The mode of operation is also an important factor. Those that function in a pulse mode measure each individual interaction (radiation hitting the detector). By contrast, in current mode, the electrical signals generated from individual interactions are averaged together, forming a net current signal (ion chambers). Pulse mode devices are less effective at measuring higher radiation levels. Under such circumstances, current mode instruments are best utilized.
 Since alpha and beta radiation can be biologically damaging it is critically important not to ignore them in any public safety or health physics program. While the cost is high to detect them with specialized scintillation or silicon detectors, specialized Geiger tubes fitted with mica windows are a good choice in most applications. 
The venerable pancake Geiger-Mueller radiation detector is utilized as a standard in most hospitals, laboratories and public safety programs because it has the ability to detect all four major types (alpha, beta, gamma and X-ray) of radiation in once cost-effective device. 
In summary, there is a wide array of technologies utilized in radiation detection, each with its benefits and disadvantages. For most practical application the Geiger counter, based upon Geiger-Mueller tube technology, will likely remain the most commonly utilized because of its combination of ruggedness, low price and adequate sensitivity.