Parasites differ from parasitoids in their relationship with the host. In a truly parasitic relationship, the parasite and host live side by side with little or no damage done to the host organism and the parasite taking enough nutrients to live on and reproduce without draining the hosts reserves. In a parasitoid relationship, the host is usually killed after the full development of the other organism. This types of relationship only seems to occour in organisms that have fast reproduction rates (like insects or mites).

There are four groups of insect that are renowned for this type of lifestyle. Three are well known and contain a huge number of species between them, while the fourth, the Stylopid (or stylops) are in fact closest to the Coleoptera (Beetles), and are less well known.

Below are the four groups (two in the Hymenoptera):