The introduction from Listverse:

A transit in astronomical terms is when one heavenly body passes in front of another such that, as viewed from Earth, we can see one move across the other in the background.

The moon transiting in front of the sun during a solar eclipse, for example. Much more rare than a solar eclipse is the planet Venus transiting the Sun. The last time this took place was in 2004. But you are in luck! The next transit of Venus will occur this year! On June 5 to June 6, 2012 those positioned in the right spot on Earth, and with clear skies, will get to see this very rare event.
The best spot to see the transit will be in the Pacific Ocean. The island of Tahiti is ideal for those who wish to travel to see it happen and the island is making preparations for many “astronomy tourists” to go there to view the transit. Portions of the transit will be viewable from Europe and North America. Most of South America and western Africa will not be able to see the transit. What an observer will see is a small black dot (Venus) passing in front of the sun. Depending on where you are on Earth to view the transit, you may see the dot move slowly across the Sun for several hours.

http://listverse.com/2012/02/12/10-amazing-facts-about-the-transit-of-venus/