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If Venus and Earth orbited the sun on the same plane, transits of Venus would happen five times every eight years. That's because Venus swings to inferior conjunction - more or less in line with the Earth and sun - five times in this eight-year period. See Diagram of Venus' orbit around the sun at right.

Venus' orbital plane is inclined to Earth's orbital plane by 3.4o. For half of Venus's orbit, Venus travels north of the Earth's orbital plane, and for the other half of its orbit, Venus travels south of the Earth's orbital plane.

Two points on Venus' orbit cross the plane of Earth's orbit at points called nodes. If Venus is traveling from north to south, it's called a descending node. If traveling from south to north, it's an ascending node.

Although Venus passes in between the Earth and sun (at inferior conjunction - see above diagram) five times every eight years, Venus more often than not swings to the north or south of the sun's disk. But this time around, Venus at inferior conjunction coincides very closely with its descending node, to stage a rare transit of Venus.
As seen from the Western Hemisphere (North America, Central America, northwestern South America and Hawaii), the 2012 transit of Venus will occur in the afternoon hours on June 5. From the world's Eastern Hemisphere, it'll take place in the daylight hours on June 6.

Everything you need to know: Venus transit on June 5-6

Tony Misch and William Sheehan: Video of 1882 Venus transit