The Major Arcana are the most well known part of a Tarot deck. In fact, most people seem to assume that they are all there is to a Tarot deck at all. In truth they are no more important than the Minor Arcana, but they are more powerful. I think of them as knots on the tapestry of life, even if you don't bump into them, they have a powerful effect on the tapestry as a whole.
The Major Arcana are numbered from 0 to 21, and represent a journey through life to enlightenment. The first card is numbered 0 as it comes at both the beginning and the end. When we begin the journey we are naive and open to possibilities. When we finish the journey we have recaptured our simplicity and purity and are now open to the full potential of the universe.
The details of the journey have been described far better than I could in the books noted in the 'Links' section of this site. However, as you read the details of the cards you should see how the pattern develops.