Castile soap are made with Olive oil. Depending on the Castile soap and who's making it, you either have a lot of olive oil, or it's mixed with cheaper oils like coconut and other vegetable oils. The olive-oil only soaps are typically costlier.
As for use, people typically use them on their body, hair, and even in cleaning. If you would like to buy some of the Castile soap you saw, you would probably have to test its strength or concentration by just seeing the way it washes your hands. If a few drops does the trick, you may need to dilute it for larger uses. If you have to use a lot more, then use it like you would standard liquid soap.
Castile soaps are considered healthy because there aren't many people on the planet allergic to Olive Oil. Olive Oil does not clog pores (is non-comedogenic) either.