In a symbiotic relationship, the smaller creature involved is known as the symbiont. Some bacteria within the human digestive tract are a good example, as these bacteria actually aid in the digestion of the human's food. Here the relationship is beneficial for both host and symbiont: The bacteria feed off some of the human's food and the human can more easily digest the food for him/herself thanks to the bacteria.
It is not required that symbionts live within their hosts, nor even be of miniscule size like bacteria. An example of a larger, less biologically intrusive symbiont is the remora. Remoras are small fish that cling to the bodies of large sharks and feed upon the smaller leftovers of their hosts' meal. Remoras also feed upon some of the parasites that attempt to harmfully feed off sharks. Again, the relationship is beneficial for both parites: The remoras get easy meals and the sharks get to stay clean and parasite-free. This relationship doesn't always work out, however, as remora remains have been found in the belly of some sharks.
In both the bacteria/human and remora/shark examples, it can be seen that symbiosis can have positive effects for both symbiont and host. Older definitions of a symbiotic relationship excluded the relationship between a host and a parasite, as such a relationship caused harm to one of the creatures involved. A more recent definition includes parasites as symbionts as well, despite their non-beneficial nature.