A parasite is a creature that invades a host and provides nothing to the host in return. The Goa'uld are parasites, in that they forcibly take control of their hosts. The host's consciousness remains intact but is forced into the subconscious. This experience is torterous for the host and is most evident when the host's consciousness emergies briefly. The Tok'ra are also parasitical in that they need hosts to live, but they are much more beneficial to their hosts, and only take willing hosts (in which case, the Tok'ra are more a mutualistic species than a parasitic species, as they benefit both themselves and their host).
A parasite exists in the Pegasus galaxy that attaches itself to the brain and causes what is known as "Second Childhood". It takes only a few weeks to grow exponentially in size, and is usually fatal to the host. However, ionized radiation, discovered at the Shrine of Talus, is harmful to the parasite. It will instinctually shrink into itself and detach itself from the brain in order to protect itself from the radiation, at which time it can safely be surgically removed from the brain of the host. (SGA: "The Shrine")