When Venus passes directly between earth and the sun, we see the distant planet as a small dot gliding slowly across the face of the sun. Apparently, it was used to measure the distance from the Earth to the Sun (But, that's a little to clever for me;-)
The coming transit of Venus on June 5-6 2012 is a rare astronomical phenomenon that will not be repeated until the year 2117. Previous transits occurred in 1631, 1639, 1761, 1769, 1874, 1882, and 2004. This page contains a diagram of the fast upcoming future transit of Venus.
These are some web sites with more information about transits of Venus:
Two comprehensive web sites on the transit of Venus are http://www.transitofvenus.nl and http://www.transitofvenus.org.
General information about the science and history of transits of Venus can be found at the wikipedia entry,http://en.wikipedia.org/wiki/Transit_of_Venus