Can you see that mole on Sun’s face? That’s Venus passing through the Sun!! Transit of Venus simply means passing of Venus between Sun and Earth.We know that the only planets that can ever pass between the Earth and the Sun are Mercury and Venus (since they are the only planets closer to the Sun than the Earth is). Whenever this happens that a smaller body (like Venus) passes between the Earth and the Sun, its called a transit.

But why is this a very rare event? Look at this video to understand:

It is an extremely rare event because each planet has different orbit size and angle. Also each planet circles the sun at different speed from the other planets.
Venus passes between earth and sun every 105 years. Two transits of Venus occur 8 years apart every 105 years. The first transit of Venus occurred in June 2004. 8 years later second  transit is this morning on June 6,2012.

When transit of Venus occurs, Venus appears as a small, dark object crossing the Sun. This Phenomenon lasts for about 3 to 7 hours. The spectacle is visible in India between 5.42am and 10.22am today.

The next one will occur in June 2117!! Imagine how old you will be?? So do catch this one!!

Important Note: 
Dont look at the Sun directly during the Transit. This can damage the eyes. View the Transit of Venus through safe solar filters.