Made from herbs, fruits, seeds or roots steeped in hot water, herbal teas have lower concentrations of antioxidants than green, white, black and oolong teas. Their chemical compositions vary widely depending on the plant used.
Herbal tea isn't really made from tea - which is a specific kind of plant. The French use the word tisane, which is a little more accurate since herbal tea is really just an infusion of leaves, seeds, roots or bark, infused in hot water. In drinking a well-steeped herbal tea, you get all the plants benefits in an easily digestible form.
If you're drinking tea for the medicinal benefits, then definitely steer clear of products that contain additives like artificial flavours or flavourings. To really get the full benefit from drinking herbal tea, make sure you steep your loose tea or tea bags long enough - in some cases, as long as 10-15 minutes - to really bring out the health properties.