Scientists believe that Jupiter is mostly composed of hydrogen (90%) with some helium (10%) and various trace elements, but it's so big that atmospheric pressure gets really high, really quick. This causes the hydrogen to turn into a fluid and eventually the electrons get squeezed out, turning it into metallic liquid hydrogen, which is conductive. That creates a massive magnetic field. Where this magnetic field (magnetosphere) interfaces with the solar wind from the sun is the bow shock.
Similarly, there's a bow shock for our solar system where the sun's magentic field (heliosphere) gives way to the galaxy's magnetic field. Voyager 1 crossed the bow shock a few years ago, I believe, and it behaved very differently than scientists had surmised.
Anyway, neat stuff. I love science. Juno also became the fastest man made object in history as it accelerated toward Jupiter's gravity well prior to firing its engine. It got up to 165,000mph relative to earth (265,000 km/h).
Regarding science, it's got a bunch of instruments to measure the gravity field, magnetic field, presence of water, and multiple cameras, including ultraviolet and infrared ones.