Reverse culture shock is when a person returns back to their home country, after being in another country for some time and they experience a hard time adjusting back. Two reasons why this may happen is when there's an idealization of their home country, and expecting things to be the same as they were prior to leaving their home country. Some of the symptoms are feeling isolated, bored, and being misunderstood. Culture shock is in a foreign country, while reverse culture shock may happen in the country of origin after returning from another country.

Michael is correct in that it is experienced by those returning to a culture, however it is not always after a many years. A colleague at University experienced it when going home for the summer and faced both societal and family restrictions.

"culture shock" is the shock you experience when you arrive in a different culture and so much is new and unfamiliar. The term "reverse culture shock" is not a standard expression, as far as I know, so it would help to know the context. It could mean the experience that an expatriate has when they return to their home country after many years abroad, and find it so different from the time that they left it.