When a planet passes in front of another planet, star, or the sun, the event is called a transit. Most commonly, the inner planets Mercury and Venus transit the sun. Less often, a planet passes in front of another planet.
Mercury transits the sun about 14 times per century. The events always happen in May and November. In those months, Earth is located at close to the places where its plane of orbit intersects with Mercury's. Over the course of many thousands of years, these transit times will shift to other months as the two planets' orbits drift.
Transits of Venus occur in pairs. These pairs begin about 113 to 130 years apart. Because more than a century can pass between events, a person can live a very long life without ever seeing Venus transit the sun. Most people who saw the transits of 2004 and 2012 won't be alive to see the next ones in 2117 and 2125.Seeing a planet transit the sun requires a sun filter or a solar image projection method like those used for safe viewing of solar eclipses. Probably, you will want to magnify the image in a telescope. A filter will block the infrared, ultraviolet, and most of the visible light so the telescope won't be damaged and your eyes will be protected. Also, an image of the sun can be projected onto a screen, a wall, or a sidewalk through a pinhole for safe viewing.
The last planet to transit another planet was Venus passing in front of Jupiter in 1818. The next such event will be in 2065 when Venus will again pass in front of Jupiter.
Here are when Mercury and Venus transit the sun in the 2010 decade. There will be no more transits of Mercury until 2032 and no more of Venus until 2117.
Transit of Venus on June 6, 2012 at 01:30