Transits happen when one object moves in front of another. This can happen when a Planet or Asteroid passes in front of a Star or when Planets appear to pass over each other.
A transit of Venus took place on 2004 June 8th between 05:20UT and 11:20UT when it crossed the Sun. I was at Worth Matravers, on the South coast of England. I used my LX200 with f/6.3 reducer and ToUcam, with a full aperture solar filter to get the high magnification images of ingress and egress. I also used a 300mm SLR lens with filter to image as much of the Suns disk as possible.
Below are links to a sequence of images showing ingress and egress of the transit.