As seen from a planet that is superior, if an inferior planet is on the opposite side of the Sun, it is in superior conjunction with the Sun. An inferior conjunction occurs when the two planets lie in a line on the same side of the Sun. In an inferior conjunction, the superior planet is "in opposition" to the Sun as seen from the inferior planet.
The terms "inferior conjunction" and "superior conjunction" are used in particular for the planets Mercury and Venus, which are inferior planetss as seen from the Earth. However, this definition can be applied to any pair of planets.
A planet (or asteroid or comet) is simply said to be in conjunction, when it is in conjunction with the Sun, as seen from the Earth. The Moon is in conjunction with the Sun at New Moon (or rather Dark Moon).