Render Mode
Has four modes:

Motion Preview - draws dots at each particle, a quick way to preview the motion of particles.

Full Render - the full monty (except for DOF)

Full Render + DOF Square (AE) - Full render with Depth of Field. This mode uses the same square aperture that AE uses internally for DOF. Useful when other elements are treated with AE's own DOF.

Full Render + DOF Smooth - Full render with Depth of Field. This mode gives a smoother DOF effect that in most cases look more realistic. It is slightly slower than the square mode.

NOTE that only Custom Particles are affected by the choice between Square and Smooth DOF, the primitive particle types Sphere, Cloudlet and Smokelet always use a circular aperture. DOF does not work at all with the particle types GlowSphere and Star.

More on how to use Depth of Field