Infrared (IR) light, or more precisely infrared radiation, is an electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye but can be perceived in the form of thermal radiation. Fun fact: more than half of the energy radiated by the sun reaches the earth in the form of infrared.
With IR it is possible to find out what sample is made of, but also how much of a certain ingredient or component is present. Qualitative analysis is the most common application of IR spectroscopy and is mainly used in quality control of raw materials, failure analysis, and in scientific research. Quantitative analysis is widely used in industrial processes to evaluate production parameters.
Definitely not. IR spectrometers are easier to use today than ever before. Most of the time there are simple software solutions (e.g. touch operation) that allow non-experts to perform IR analyses in an uncomplicated way. Even the analysis can be automated, so anyone can become a spectroscopist!
In general, yes, because organic and inorganic substances can be examined equally well with infrared radiation. The basic requirement for analysis with infrared is that the material absorbs infrared radiation. Certain substances, however, including metals and monatomic gases (e.g. noble gases) cannot be examined directly.
Especially for organic substances IR spectroscopy is a frequently used tool to obtain a lot of information. This includes the identification of polymers, drugs, pharmaceuticals, or industrial chemicals as well as the determination of contents like water in oil. IR spectroscopy is very flexible, and its applications are so numerous that you can find IR users in all industry and research areas.