The Fool is an oddity in the Major Arcana, because unlike his compatriots, he is numberless, and represented by a zero. In medieval times, the Fool was known as a jester, an innocent in the eyes of the gods, and as such he could get away with saying all kinds of inappropriate things. In fact, the Fool can say or do as he chooses. The Fool is traditionally portrayed as a young man, often a bit on the androgynous side. In the Rider Waite deck, he is a courtly and almost foppish young man in a tunic and tights, preparing to set out on a journey. He looks exquisitely happy, almost as though he's daydreaming, and yet doesn't seem to notice that he's about to step off a cliff.
In fact, when the Fool comes up in a Tarot reading, it's a sure sign that a new adventure or journey is about to begin. The Fool signifies a time of big changes, and big adventure. The Fool is the agent of fortune, and despite the possible risks or unknown that lies ahead, it's worth it to follow one's heart and accept the new changes.
When reversed, the Fool indicates a tendency to "look before you leap". Think before you act, and accept that a lack of attention to detail could lead to costly mistakes later on. This can also indicate a reluctance or refusal to accept personal responsibility.