tough question. INFPs can be surprisingly J-ish at times, many of them are perfectionist (in a more subjective, chaotic, and disorganized sort of way) and if you get in the way of a moral cause, they will kill you (although unlike an ENTJ, they will feel bad afterword =)
likewise, INFJs can be surprisingly P-ish at times. they tend to be more open minded than most Js and have more perceiver interests (music, art, humanities) and can be extremely creative.
they have a lot of similarities: introspection,intense idealism, creativity, open mindedness, supreme intelligence, good at counseling people, great as friends (IFs in general are in my opinion the best types for friends)
the difference is mostly found in Fe/Fi. INFJs have Fe which is more people oriented. FJs help people because they feel like they have to. FPs help people because they want to and feel like it. while FPs thrive on helping others, they are by no means selfless. an INFJ will sometimes completely neglect their own needs and tear themselves apart via service. Fi makes sure this does not happen to INFPs who know that there self takes #1 priority (they are incredibly in touch with their feelings). INFPs help themselves first and help others once they have grown enough to do so (they want everyone to win but are willing so sacrifice much less than their J cousins)
at their best their differences come out more. INFPs will be happy and bubbly, full of imagination and internal energy (which they are much more willing to share than many introverts. an INFP once comfortable makes for an extremely affectionate friend of great conversation). although they are introverted, the NP (Ne) in them loves to talk. while INFJs are also creative, NFPs are by far the most creative 2 types out there (in fact, it's almost as much a curse as a gift. I enjoy this frequently, but as an entrepreneur, it often gets in the way of effectiveness, despite having lent me many profitable ideas).
INFJs on the other hand are probably the most intelligent of the types (well, maybe save INTJ), when they really trust their Ni, they are almost psychic. even so, they tend to be way to serious for my tastes and even at their best they play the broken hero who feels they need to sacrifice everything.
just a few examples of each
INFP: most authors, Johnny Depp, Fai from Tsubasa Chronicle, Star Girl (from the book of this title), the Little Prince, Bastian from the Never Ending Story
INFJ: Kenshin from rurouni kenshin, Aragorn from Lord of the Rings, Martin Luther King Jr., Ghandi, Yugi from Yugioh, in ancient times: most shamans, oracles, and seers,
a few I'm on the fence about: Joan of Arc, Mithos from Tales of Symphonia, Amelie