Price discrimination is the practice of charging different buyers different prices for the same product, even when there are no cost differences. (Note, the word "discrimination" has a negative connotation but price discrimination can actually improve the well-being of society. Further, military and senior discounts are forms of price discrimination.)
Here is a picture of cruise pricing on Orbitz.com All prices are for the same cruise, just different dates.
The cruise industry also engages in other forms of price discrimination. I'll post about those soon.