Particular's particles are emitted and temporary.
Form particles are constant and static.
Particular is built for creating animated particles with photorealistic results, such as creating explosive fireballs or fireworks where the particles change over time.
Form is a 3D surface generator where a surface can be made from particles, but the particles are static (they don't have a life span where they live and die). Form surfaces can then be animated by using built-in maps, fractal distortion, dispersion, displacement, audio and more.
In short, Form uses particles to generate the appearance of a 3D surface (like a sphere, grid or terrain) with unique effectors that animate the surface. Particular focuses on creating 3D volumes of particles for photorealistic effects such as fire, sparklers and more.