The short answer
BR30 produces a broad, soft-edged flood. PAR30 produces a more controlled, higher-intensity beam.
SpecificationBR30PAR30
Nominal diameter3.75 in3.75 in
BeamWide and softFocused and defined
Best forGeneral recessed lightingAccent and task lighting
Common baseE26E26
Before you substitute one for the other
Also check neck length, can depth, wet-location rating, and beam angle.