A particle is a free 3D coordinate. Particle rendering refers to several visualization techniques to visualize a volumetric system which focus on minimizing per particle work load:

https://en.m.wikipedia.org/wiki/Particle_system

A voxel is a value in a 3D lattice.

https://en.m.wikipedia.org/wiki/Voxel

A particle system could be used to visualize a voxel field, of course, but their application range is far wider.