The term is most frequently used to describe the motion of a planet beween a terrestrial observer and the Sun. This can happen only with inferior planets. To a pedant, an annular eclipse is a transit of the Moon.
In recent years the discovery of extra solar planetss has excited interest in the possibility of detecting their transits across their own stellar primaries.
Transits of Venus with respect to Earth are rare, they currently happen in pairs 8 years apart which repeat only after more than a century. The last transit of Venus was in 1882; the next three will occur on June 8, 2004, June 6, 2012, and December 11, 2117. In the 18th century, the transit pair of 1761 and 1769 was used to determine the value of the astronomical unit.
Transits of Mercury with respect to Earth, being closer to the Sun, are more frequent, about thirteen per century. The last ones were on November 15, 1999, and May 7, 2003, the next ones will be on November 8, 2006, May 9, 2016 and November 11, 2019.