OURA ring - heart rate variability and sleep tracking
Being a ring, the OURA is in direct and constant contact with the finger's arteries (vs. a loosely sitting wrist strap). Because of this, the OURA is able to receive ongoing high fidelity data. For example, the ring tracks heart rate variability (HRV), which is the measure of variation (time) between beats, no Fitbit or Apple Watch today offers this functionality. Professional sports teams are starting to track HRV in their athletes to gauge fatigue. For the consumer, this means you receive a daily "readiness" score letting you know how hard to push yourself.
The OURA also tracks respiration (although this isn't exposed in the app), body temperature, and resting heart rate. This allows them to more accurately gauge your sleep state. I recently discovered I'm lacking in deep sleep, so that's an issue I'm working on fixing now.