What are absoluteExpiration and slidingExpiration parmeters of the Insert and Add methods?

SlidingExpiration
        A TimeSpan object that identifies how long the data should remain in the cache after the data was last accessed.
        If you’re using absolute expiration, specify Cache.NoSlidingExpiration for this parameter.
AbsoluteExpiration
        A DateTime object that identifies when the data should be removed from the cache.
        If you’re using sliding expiration, specify Cache.NoAbsoluteExpiration for this parameter.