HRVs and ERVs exchange stale indoor air with fresh outdoor air.  While doing so, they also exchange heat energy in the two parallel airstreams to minimize the amount of conditioned air that gets exhausted from the home.

An ERV recovers both heat and some of the moisture in the air to keep it on the same side of the thermal envelope that it came from. So, in winter, the system transfers the humidity from the air being extracted to the incoming fresh (and dry) air to help keep the ambient humidity level at a comfortable level all times. In summer, the humidity transfer reverses and the humidity in outside air is removed before it is brought into the home. This saves energy by reducing the load on your air conditioning system and/or dehumidifier. A high efficiency of humidity transfer would be around 70% but this value depends on the actual humidity on either side of the envelope.

The best option between an HRV and an ERV depends on your climate and specific needs. If your house is too humid in winter (above 60% RH) then an HRV is the better choice, as it would surely get rid of excess humidity while an ERV would tend to keep it at a high level. If the opposite is true and your house is too dry in winter, then an ERV would be a better choice as it helps retain humidity, eliminating the need (and cost) for you to generate it through other means.

In summer time, the use of an HRV will usually increase the humidity level inside your home, so an ERV is better in hot and humid climates.