An opposition occurs in a chart when two planets are 180 degrees apart, or in direct opposition to each other (the circular horoscope is made up of 360 degrees). The acceptable orb of influence for an opposition is eight degrees. This is one of the major Aspects, and a hard energy is generally ascribed to it.
This Aspect is the widest one to be found in a horoscope. We also tend to see polarities in an opposition, such as Aries to Libra or Virgo to Pisces. The planets in an opposition may be from different yet complementary Elements (say Gemini's Air to Sagittarius' Fire), and they may also be from the same Quality, be it Fixed, Cardinal or Mutable. These traits, when present, make for some common ground between the planets in an opposition.
More often than not, however, an opposition is a poster child for conflict resolution. This Aspect, as its name would indicate, presents opposing forces. Mars' energy may find itself at war with Venus' golden glow, for example. The key here is for the individual to see the conflict, the challenge at hand, and to find a way to act upon it. For that reason, many people prefer to view an opposition with an eye to the opportunities it can bring. With a heightened awareness of conflict, we can take the steps necessary to achieve balance and harmony.
The key to conquering an opposition is to bridge the great divide. The opposing energies are far apart only until we have brought them together.