Oppositions (180 degree angles) challenge your relationships. Sometimes it's positive, stimulating an exchange of energy, while other times, oppositions may bring conflict, confrontation or separation. Or, someone else could be mirroring something about yourself that you need to deal with (but have had difficulty seeing).
Opposing planets may have the same intention, but have two very different ways of getting what they want. However, they can find a middle ground and possibly work together, unlike some of the more difficult "incompatible" aspects.
Aspects are angles created by the varying positions of the planets in the sky in relation to one another.