A transit occurs when an object in space passes in front of another one. An example of a planetary transit occuring is, when viewed from the Earth, the planet Venus passes in front of the Sun's disc and can be seen as a small black disc on the Sun's white disc.
Transits can also be seen with the planet Mercury passes in front of the Sun. This is because the orbits of Mercury and Venus are inside that of the Earth so at certain times those planets will pass between the Sun and the Earth. Transits of Mercury or Venus cannot occur every year however, this is because the planets orbits are on a different plane to that of the Earth's, in other words they only line up at certain times.
The orbits of Mars, Jupiter and Saturn lie beyond that of the Earth so a transit with the Sun cannot occur since those planets never pass between the Earth or the Sun.
However, with a telescope we can see a transit occur when the moons of Jupiter or Saturn pass in front on those planets.
Eclipses are similar to transits except that the object in front completely covers (or significantly covers, in the case of a partial eclipse) the object furthest away. For example the Moon is said to eclipse the Sun when it passes in front of the Sun's disc. The Moon appears so much larger when it passes across the Sun's disc compared to Venus because it is so much. closer.
An occultation is also similar the difference being the object in front appears to be larger and will partially cover the object further away. For instance the planet Saturn passing behind the Moon.