Very often we consider the changes in demand of a good from changes in the prices of the same good. When the changes and effects we consider is related to the same good, we are working with own elasticities.
When we consider the effects on one good from changes in another good, we are considering cross-elasticities. This can be the change in demand for public transport when the costs of car use increase. When the cross elasticity of fares is positive, that is when increased cost of car use increase the demand for PT, we have competing goods. In the opposite case, we have complementary goods.
In some relations we consider the effect from symmetric changes for different goods. In that case we deal with conditional elasticities. This can be when we consider the changes in demand for metro from a symmetric change in the fares for all PT modes. In general, the sign of this will be the same as for the cross elasticity, but the magnitude will be much smaller.