IV Rank and IV Percentile both look at the current level of Implied Volatility but they will tell you different things. IV Rank will tell you the current level of IV compared to the high/low over the last year. IV Percentile will tell you the frequency of that level.

IV Rank

The IV rank compares the current IV to the 52-week high and low IVs.  

IVR = (high IV - current IV) / (high IV - low IV)

IV Percentile

The IV % compares the number of days in the past year when it was lower than the current IV to the total number of days.

IV% = # days when IV was lower than current IV / 262