HCA stands for hydroxycitric acid and is just one of the natural extracts contained in the product(others include Gorikapuli and Gummi-gutta). HCA is considered the active ingredient in GC as it has potent effects in the body that help suppress the appetite, increase metabolism, burn fat, and prevent carbohydrates from converting to fat once inside the body. Due to its high-quality makeup, Pure Garcinia is also by far the most popular product.
It is 100 pure with no adulterants or fillers and also ships the fastest in Canada. The best place where you can buy it is the site linked to above.