Flagellates are a paraphylogenenetic group of parasites. Paraphyletic refers to a group of taxa that includes some, but not all of the decesdents of a last common ancestor. The unifying characterisitic of this diverse group of parasites is the presence of one or more flagella. These organisms typically reside in the bloodstream or intestine of their hosts.
Flagellates, such as Trichamonas vaginalis and Giardia lamblia, have simple trophozoite (internal feeding stage) to cyst (external stage) life cycles. These life cycles are simple because invertabrate vectors are not involved in the process. Flagellates such as the Leishmania spp and Trypanosoma spp, have more complex life cycles because they go through stages in the gut of the invertebrate host before they are transferred into a vertebrate host, where they begin a whole different series of stages in various parts of the vertebrate. An example of the typical Trypanosoma life cycle is presented in the upper right corner of this wikia page. This particular parasite is Trypanosoma cruzi, which causes Chagas disease.
One of the more distinct characteristics of the Flagellates is that they contain a flagellum. A flagellum, which is latin for "whip", is a tail-like structure that helps to propel the cell body.
One group of flagellates, the Kinetoplastids, have stages that are often referred to by the attachment and direction of their flagella: trypomastigote, epimastigote, promastigote, and amastigote refer to different stages of flagella retraction.