What information is stored in checkpoint or snapshot files?

To find out what each of the fields are in CheckPointer output files, you need to look at Model/Particle.cpp and Model/RotParticle.cpp, specifically the saveCheckPointData(...) subroutines in those files.

For NRotSpheres, the fields written out for each particle are:
Position X,Y,Z
Radius
Particle ID
Particle Tag
Mass
Initial Position X,Y,Z
old Position X,Y,Z
Linear Velocity X,Y,Z
Force X,Y,Z
circular boundary shift X,Y,Z

For RotSpheres, the following additional fields are also written out for each particle:
Moment of Inertia
orientation Quarternion Q0,Q1,Q2,Q3
Angular Velocity X,Y,Z