The word "parasite" refers to any number of biotrophic organisms, organisms that feed off of a host without killing them, ranging from microscopic bacteria to macroscopic leeches. Parasitic creatures that eventually cause their host to die or become sterile are not parasites, they are called parasitoids and are necrotrophic. Parasites live off of a host, and keep that host alive so that they can continue to benefit. Parasitoids on the other hand generally feed off of their hosts while they're young, killing the host in the process. Parasitoids become free living in their adult stages, and no longer have a need for a host. The line between parasite and parasitoid is somewhat blurry.
There are many different types of parasites, like protelean parasites that are only parasites during their juvenile stage. Parasites classified based on if they live inside or outside the host are endoparasites and ectoparasites, endoparasites living on the inside of their hosts body, and ectoparasites living on the outside of their hosts body. Endoparasites are further classified into intercellular and intracellular parasites. Intercellular parasites live in spaces inside the host body, such as the intestines or cavities that the parasite makes for itself. Intracellular parasites live within the hosts cells themselves, this classification includes various bacteria and viruses. Other parasites called epiparasites or hyperparasites are parasites that live off of other parasites. It turns out that nothing is safe from being fed upon by a parasite.
Parasites do more than just feed off of a host. Social parasites include classifications like kleptoparasitism where the parasite steals food from another creature, or brood parasitism such as various species of birds that leave their eggs in another birds nest to be cared for by the host bird. Parasitism occurs pretty much whenever one creature exploits another for its own gain.
Some of the more interesting kinds of parasites are capable of infecting a hosts brain and influencing or even controlling its actions. One of the more widely known "mind control" parasites is the Cordyceps fungus which is known for infecting the brains of bullet ants and causing them to have an uncontrollable urge to climb. The ant will cling onto a branch as the fungus continues to grow, eventually killing the ant and growing out of the creatures exoskeleton to release its spores and infect other ants. There are several kinds of Cordyceps fungus, each one specialized to a different insect, and each more fascinating than the last.
Another mind controlling parasite is a barnacle called Sacculina that infects crabs. A female Sacculina barnacle will find a joint in a crabs armor and then molt out of its shell, injecting itself into the crab. While inside the crab, the parasite will grow tentacles through the hosts body, altering the crabs hormone levels and rendering it infertile. If a male crab is infected, its physiology will even be altered such that the crab becomes female. Once a male Sacculina comes and impregnates the female growing within the crab, the crab will care for the parasites eggs as if it were its own. The host actually cares for and nurtures more parasites.
Parasites are often disturbing creatures, but you can't help but admit that they are fascinating. They are so specifically adapted to their prey, and they are capable of amazing, and horrifying, feats.