Flavor and Chemical Composition of Thyme
Thyme is dominated by the phenolic compound thymol, with the terpenes pinene, cymene, and linalool adding additional pine, citrus, and flowery flavors, respectively. 

Source: On Food and Cooking (revised edition), pp 391, and table on 392.  If you're into the chemistry of flavor, I highly suggest buying a copy, because the same table lists main flavor compounds for dozens of common ingredients and flavorings.