The Seer of Lublin once said, "Between mentor and mentor (Rebbe and Rebbe) I can't differentiate and I don't want to differentiate. Between disciple and disciple (talmid and talmid) I can differentiate but I don't want to differentiate."
The two mentors he was referring to were the Arizal and the Ba'al Shem Tov. The two disciples he was referring to were Rabbi Chaim Vital (the greatest of the disciples of the Arizal) and Rabbi Dov Ber, the Magid of Mezritch (the greatest of the disciples of the Ba'al Shem Tov).
The classic example of a mentor and disciple in the Torah is Moses and Joshua. The relation between the two is portrayed by the sages, "The countenance of Moses is like that of the sun while the countenance of Joshua is like that of the moon."
The moon receives its light from the sun and reflects it to earth, thereby illuminating the otherwise dark night, the time that the sun and its light have disappeared from our human eyes. The night remains night (no one would confuse it for day) but thanks to the moon and its light we can nonetheless see our way around.
Likewise, a disciple reflects the light of his mentor to the world in a time or situation that the world is unable to receive the great light of the mentor directly from the source.
In the beginning God created two great luminaries, the sun and the moon, but afterwards He diminished the light of the moon so as to distinguish between day and night. The prophet Isaiah says that in the future the light of the moon will once more become as brilliant as the light of sun, which itself will be more brilliant then it was at the outset of creation.
And so, as we approach the time of Mashiach, the level of the disciple raises to approach that of the mentor (until, ultimately, the Mashiach and his disciple will be at the same level). The faithful, devoted disciple continues to receive his light from his mentor, just as the moon receives its light from the sun, but he succeeds in shining all of the mentor's light to the world (in a certain sense he even improves the quality of the light, making it more accessible to the general public).
