PTC is used as a test to determine whether the person can taste its bitter taste or not. You can buy some PTC strips here http://www.home4schoolgear.com/ptctapabphb1.html
I quote the following from http://www.ncbi.nlm.nih.gov/pubmed/11293722 which should help you understand more
"The ability to taste the bitter compound phenylthiocarbamide (PTC) and related chemicals is bimodal, and all human populations tested to date contain some people who can and some people who cannot taste PTC. Why this trait has been maintained in the population is uncertain but this polymorphism may influence food selection, nutritional status or thyroid metabolism. The gene product that gives rise to this phenotype is unknown, and its characterization would provide insight into the mechanism of bitter taste perception. Although this trait is often considered a simple Mendelian trait, i.e. one gene two alleles, a recent linkage study found a major locus on chromosome 5p15 and evidence for an additional locus on chromosome 7. The development of methods to identify these genes will provide a good stepping-stone between single-gene disorders and polygenic traits."