sf stands for sforzato while sfz is for sforzando. The first being a past participle and the second a present participle. I don’t know anyone who makes a distinction between the two marks.
There are some purists, such as John Harbison, who always put sf within a context by notating a prevailing dynamic below or next to it:
sf
p
or psf (this latter I find confusing).
Some organs have a sforzando piston which adds many extra ranks for a sudden increase in volume. This might suggest that at one point sf was for a single accented note and sforzando was for a sudden and continuing increase in loudness, but, of course this distinction is lost now.