The adjective parasitic is mainly a scientific term for talking about an organism that lives on a host, taking what it needs to stay alive while often injuring the host.
By their nature, ticks, leeches, and lice are all parasitic; they live off their hosts. You can also use the word parasitic more metaphorically, to describe a person who takes without giving anything in return. A thirty year-old man who lives with his mother, eating her food and not paying rent, could be described as parasitic, since he survives by sponging off another person. Parasitic comes from the Greek word parasitos, "eating at another's table."