"Percentage" is a commonly used word, and it means a fraction expressed a hundredths. It is often expressed by using the percent sign, % "This drink is only 10% juice and 90% water." "When you flip a coin, it has a 50% chance of coming up heads." "In Iceland, 99% of the population is literate." "Percentile" is a technical term in statistics, expressing the relative position of something when a group is ranked by size. "He scored at the 68% percentile" means that 68% of those tested scored lower and 32% score the same or higher.

A dictionary should be your first stop. http://www.thefreedictionary.com/percentile http://www.thefreedictionary.com/percentage You'll see that one of these words has a very specific usage.