Hi monstersaurous,
the true equation for friction is
f ≤ μR
as you have said, R is the normal reaction force :)
As for μ (pronounced as μ), it is called the coefficient of
friction. The ≤ sign is because friction acts to oppose, up to the
point of maximum friction where f = μR.
For μ, we can further divide into μs and μk.
μs is the coefficient of static friction, applicable when
objects are stationary.
μk is the coefficient of kinetic friction, applicable when objects
are moving. The ≤ sign is not used.
μs is always larger than μk because when moving, the atomic
surfaces have yet to settle fully... Hard to explain with only
words and no diagram...