Our tradition offers up contradictory reasons regarding why Chanukah is celebrated for eight days.
The Mishnah (the Oral Law - written down in the beginning of the third century C.E.) states that the holiday is celebrated because of the miracle of the oil. Yet the book of Maccabees - the account written by those who participated in the rebellion against the Greeks - doesn't mention the miracle of the oil at all.
According to Books I and II of the Maccabees, the Maccabees decided to celebrate the holiday for eight days because they were unable to celebrate the holiday of Sukkot in the fall at the Temple, since, at the time, the Temple was being used for pagan Greek worship.
As Sukkot is an eight-day holiday in which sacrifices are offered in the Temple, the Maccabees also celebrated their new holiday with eight days of sacrifices in the newly rededicated Temple.
The Mishnah's emphasis on the miracle, rather than on the creation of the holiday by the Maccabees, shows that the rabbis clearly wanted to downplay the acts of human heroes in the Chanukah story and emphasize the acts of the divine hero, the Master of the Universe. For that reason, the Haftorah (reading from the Prophets) for the Sabbath during Chanukah is from the book of Zechariah (4:6): "Not by might and not by power, but by My spirit alone, saith the Lord of Hosts."
Therefore, the lights of Chanukah represent the spirit of the spiritual fight against oppression. The lighting of the chanukiah is a symbol of pride.
In the Talmud, we are taught to light and display the lamp so that everyone will know of the miracle of Chanukah.
In ancient times, people hung their Chanukah lamps on the outside of their doors as a public demonstration of pride. Today, many Jews light their lamps in their windows for all to see.