In fact, if I tell the truth, the Mona Lisa was strange to me always, that what is in this painting that make it so popular. It was always a question for me, until I saw the painting in reality. I asked a lot of friends about this painting, they have different opinions, but the best and perhaps the main fact is written in Wikipedia:
The Mona Lisa is a half-length portrait of a woman by the Italian artist Leonardo da Vinci, which has been acclaimed as "the best known, the most visited, the most written about, the most sung about, the most parodied work of art in the world". The painting, thought to be a portrait of Lisa Gherardini, the wife of Francesco del Giocondo, is in oil on a white Lombardy poplar panel, and is believed to have been painted between 1503 and 1506. Leonardo may have continued working on it as late as 1517. It was acquired by King Francis I of France and is now the property of the French Republic, on permanent display at the Louvre Museum in Paris since 1797.
The subject's expression, which is frequently described as enigmatic, the monumentality of the composition, the subtle modeling of forms, and the atmospheric illusionism were novel qualities that have contributed to the continuing fascination and study of the work. Read more in Wikipedia