Parasitoids differ from parasites in their relationship with the host. In a truly parasitic relationship, the parasite and host live side by side with little or no damage to the host organism while the parasite takes enough nutrients to live on and reproduce without draining the host's reserves. In a parasitoid relationship, the host is usually killed after the full development of the other organism. This type of relationship seems to occur only in organisms that have fast reproduction rates (such as 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 among 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):

• Hymenoptera
  • Chalcid wasps (family Chalcidae)
  • Ichneumon wasps (family Ichneumonidae)
• Diptera
  • Tachinid flies (family Tachinidae)
• Strepsiptera
  • Stylopid flies (family Stylopidae)