Symbiosis is classified into three types: parasitism, commensalism, and mutualism. Parasitism involves one organism benefiting at the expense of the other, like a tapeworm living in a human intestine. Commensalism occurs when one organism benefits without affecting the other, such as an orchid growing on a tree. Mutualism is a reciprocal relationship where both organisms benefit, exemplified by algae and water ferns living together. Symbiotic relationships impact ecosystems, and an overabundance of parasitism can cause problems.