Symbiosis means living together in intimate association or even union, of two dissimilar organisms.  A common example are the lichens, which are really double plants, a combination of a fungus and an alga living together, each deriving benefit form the other.

Another example is the yucca plant and the yucca moth. One cannot exist without the other. Pollination is possible for the yucca only with the help of the moth. The moth larvae, on the other hand, cannot survive without the food provided by the flower, yet there always are enough seeds left so that the plant can survive.

Antibiosis is the opposite of symbiosis, as is antipathetic symbiosis. This means that the association, as in parasitism, is disadvantageous or destructive to one of the organisms - mistletoe as an example.