In a typical heat recovery ventilator (HRV), the incoming fresh airstream and the outgoing stale airstream pass very close to each other (without touching and contaminating each other) inside a special heat-recovery core. During the winter, the warmth from the outgoing air passes through the core and warms up the incoming air. In an air-conditioned house in the summer, the warmth from the incoming air passes through the core into the cool outgoing air. An HRV tempers (preheats or precools) the incoming air. No heat is actually generated, but some of the existing heat is recycled. An HRV doesn't bring the incoming air all the way to room temperature, but the process is much more energy conserving, for example, than bringing in 0°F air in the winter and 100°F air in the summer.
Source: The Healthy House: How to Buy One, How to Build One, How to Cure a Sick One (4th edition) by John Bower.
