Repositioning cruises are one-way itineraries scheduled when ships relocate based on the season. Ships that winter in the Caribbean or Hawaii will often move to Alaska, Europe or Canada/New England in the spring, and reverse course back to warmer climes in the fall.
While not for everyone (repo cruises tend to have fewer port calls and are often longer than typical cruises) benefits include long wonderful days at sea to relax and rejuvenate, and deep discounts on the pricing.
The ports also tend to be more exotic than usual. You can visit broad swaths of the Caribbean or an entire coast line all at once; cross the Atlantic with stops in off-the-beaten-path places, like Greenland; or hit several cruise regions and even continents on one trip.