Collisions tab menu.
Check this box to enable collision constraints.
This controls how stiff the collision is.
Essentially, a dynamic 'spring' is created to keep the particles apart (keeping them from intersecting).
xpConstraints_Collisions_Stiffness_v01.mp4
This animation demonstrates how the Stiffness parameter keeps particles from intersecting at higher values.
Set as All, by default.
The alternatives are: Same Group and Different Groups.
Collisions between particles will occur regardless of what group the particles are in.