Brewing the fresh or dried stevia leaf with herbal teas results in less of a bitter aftertaste. The liquid or powdered extracts are more convenient, but may lack some of the health benefits of the whole leaf. Stevia does not caramelize like sugar, which limits its use in baking.
Do not fall for the overly processed kind of stevia you can purchase in your grocery store. That stevia has no medicinal benefit whatsoever. You can make your own extracts with the tincture method by either using fresh leaves or dried leaves that you can purchase online.