A platinum print is made by hand coating a platinum-based
emulsion onto fine art paper and exposing a negative to it.
Most black and white photo prints use a machine-coated
silver-based emulsion.   The size of the image is equal to the
size of the negative.  Platinum print making is a contact
process where the negative is placed on the hand coated
emulsion and then exposed to an ultraviolet light source
such as the sun.

A quote here from photographer John Rudiak:

"Serious photographic printmakers have kept the platinum
process alive in spite of the absence of pre-coated papers…
because of platinum's' unique qualities.   First is the inertness
of the platinum family of metals; platinum,  palladium,
iridium, rhodium, and osmium. These metals (long referred to
as noble metals) resist combining with other chemicals
(unlike silver) making prints made from them very stable.  A
platinum print will remain unchanged as long as the paper it
is printed on exists."

"Platinum prints look different than any other photographic
print.  The response to light by the platinum emulsion is much
more linear than that of silver, resulting in more even spacing
of tones in the shadows and highlights... Because the liquid
platinum emulsion is brushed directly onto the paper and
sinks in, rather than being held captive in a thin layer of
gelatin on top of the paper, the image itself has physical
depth, creating a three-dimensional quality to the print.  The
finest platinum prints appear to have light emanating from
within rather than light reflecting off of them."