"Significant Motion sensor" same as accelerometer?
The significant motion sensor is a virtual sensor using the physical accelerometer. From android.com: 


  A significant motion detector triggers when the detecting a “significant motion”: a motion that might lead to a change in the user location.


This means the significant motion sensor will only provide data if a significant motion is detected by the system (e.g. walking). The accelerometer continuously returns the accelerations for each axis.

To check out those sensors you can install one of many sensor apps. I usually use cpu-z.