Phenols. A complex group of compounds, primarily carbolic acid C6H5OH, but also other compounds. The malted grains get the peat in the form of tar particles from smoke.
Ratings are usually given for the malted grain, and is much reduced in the spirit, and reduces through the years of maturation. For example 40ppm in Laphroaig is 25ppm in the new make spirit. At 10yo it's just 10ppm, at 15yo it's 8ppm, and at 30yo it's just 6ppm.
There are different ways of measuring the 'peatiness' however, and it's not an exact science. Figures given are just approximations. The peatiness in malted grain is measured colorimetrically, and measured in the new sprit by high pressure chromatography. There are seven different methods of testing colorimetrically. With high peating levels, the results vary more wildly.