Opposition is a term used to indicate when one celestial body is on the opposite side of the sky when viewed from a particular place (usually the Earth).
An opposition occurs when the planet is opposite from the Sun, relative to the Earth. At opposition the planet will rise as the Sun sets and will set as the Sun rises providing an entire night of observation (just like with a full moon). Also at opposition the planet comes physically closest to the Earth in its orbit so it appears as large as possible. For planets outside the Earth's orbit (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), the months around oppositions are the best time to view these. Conversely, during a conjunction, a planet is in line with the sun and impossible to see at all.
A Mars year (orbital period) is equal to 2.135 Earth years. Therefore approximately every 2 earth years Mars is in opposition. When oppositions occur, it is ideal not only to observe the planets, but also to send spacecraft out to the planet. The first space craft that visited Mars was the Mariner in 1964. 2010 was the first year that NASA did not send a new spacecraft during its opposition since 1996.
Looking up in the sky each night at the same time, usually you would observe that Mars is a little further east each night compared to the constellations. However, about every two years there are a couple of months when Mars appears to move from east to west when observed at the same time (retrograde motion).
This backward or retrograde motion was mysterious to the early observers, and led to the use of the word "planet", from the Greek term which means "wanderer".
It is in a short period including the time of opposition when Mars exhibits its retrograde motion to an observer on the Earth. As the Earth moves forward in its orbit, Mars will appear to slip backward compared to its more common eastward march across the sky.
This lesson is adapted from the Kinesthetic Astronomy program developed by the Space Science Institute, 2004. Additional information on Mars' apparent motion can be found at here.