what does perc95 and all those stats functions perc*

In stats calculation, I use average avg() and median but I saw other people using "percentage Xth" like perc95(). What does it do exactly ?

see docs http://docs.splunk.com/Documentation/Splunk/5.0.4/SearchReference/Commonstatsfunctions

This function returns the X-th percentile value of the field Y, where X is an integer between 1 and 99. The functions perc, p, and upperperc give approximate values for the integer percentile requested. The approximation algorithm used provides a strict bound of the actual value at for any percentile. The functions perc and p return a single number that represents the lower end of that range while upperperc gives the approximate upper bound. exactperc provides the exact value, but will be very expensive for high cardinality fields.