The order Gentianales contains five plant families:
(how to distinguish gentians from other families)

(note - Menyanthaceae is not in the Gentianales anymore)

A large family that contains the milkweeds, plumerias, rosy periwinkle, and dogbanes. They all have latex (milky sap) and many contain poisonous alkaloids.

A tiny family of only 2 genera that includes the Carolina jessamine (Gelsemium) and tropical Mostuea.

A medium-sized family that contains the gentians. What is a gentian?
 

A rather small family that has members such as the strychnine plant (Strychnos) and Logania. The ovary is superior and the flowers are often rather small. This was previously a much larger family , but some earlier members are now placed in other families (Buddlejaceae/Scrophulariacae, Desfontainiaceae, Retziaceae/Stilbaceae).

The largest family in the order includes many well-known plants, for example coffee, gardenia, Cinchona (the quinine tree), and madder. They are known for their showy flowers with inferior ovaries and interpetiolar stipules.

References and publications:

Albert, V. A. & L. Struwe. 2002. Gentianaceae in context. Pp. 1-20. In: L. Struwe & V. A. Albert, editors. Gentianaceae – systematics and natural history. Cambridge University Press, Cambridge.

APG (The Angiosperm Phylogeny Group). 1998. An ordinal classification for the families of flowering plants. Ann. Missouri Bot. Gard. 85: 531-553.

Backlund, M., B. Oxelman, & B. Bremer. 2000. Phylogenetic relationships within the Gentianales based on ndhF and rbcL sequences, with particular reference to the Loganiaceae. Amer. J. Bot. 87: 1029-1043.

Bremer, B., R. G. Olmstead, L. Struwe, & J. A. Sweere. 1994. rbcL sequences support exclusion of Retzia, Desfontainia, and Nicodemia from the Gentianales. Pl. Syst. Evol. 190: 213-230.

De Laet, J. & E. Smets. 1996. A commentary on the circumscription and evolution of the order Gentianales, with special emphasis on the position of the Rubiaceae. In: Robbrecht, E., Puff, C. & E. Smets (eds.) Second International Rubiaceae Conference. Proceedings: 11-18, 5 figs. Opera Botanica Belgica 7. National Botanic Garden of Belgium, Meise. 432 p.

Endress, M. E., B. Sennblad, S. Nilsson, L. Civeyrel, M. W. Chase, S. Huysmans, E. Grafström, & B. Bremer. 1996. A phylogenetic analysis of Apocynaceae s. str. and some related taxa in Gentianales: a multidisciplinary approach. Opera Bot. Belg. 7: 59-102.

Jansen, S. & E. Smets. 2000. Morphology, distribution, and systematic importance of vestures in the Gentianales. In: Nordenstam, B., El-Ghazaly, G. & M. Kassas (eds.) Plant systematics for the 21st century: 277-296, 25 figs. Wenner-Gren International Series Volume 77, Portland Press, London. XIII-366 p. ISBN 1 85578 135 2

Jensen, S. R. 1992. Systematic implications of the distribution of iridoids and other chemical compounds in the Loganiaceae and other families of the Asteridae. Ann. Missouri Bot. Gard. 79: 284-302.

Leeuwenberg, A. J. M., editor. 1980. Engler and Prantl’s Die natürlichen Pflanzenfamilien, Angiospermae: Ordnung Gentianales, Fam. Loganiaceae, vol. 28b (1). Duncker and Humblot, Berlin.

Moore, R. J., editor. 1973. Loganiaceae, Apocynaceae, Asclepiadaceae, Gentianaceae. Index to plant chromosome numbers 1967-1971. Oosthoek´s Uitgeversmaatschappij B. V., Utrecht. 

Olmstead, R. G., B. Bremer, K. M. Scott, & J. D. Palmer. 1993. A parsimony analysis of the Asteridae sensu lato based on rbcL sequences. Ann. Missouri Bot. Gard. 80: 700-722.

Struwe, L. 2001. Gentianales. In: Encyclopedia of Life Science, MacMillan Publishers. 

Struwe, L., V. A. Albert, & B. Bremer. 1994 [1995]. Cladistics and family level classification of the Gentianales. Cladistics 10: 175-206.

Struwe, L. & V. A. Albert. 1997. Floristics, cladistics, and classification: three case studies in Gentianales. Pp. 321-352. In: J. Dransfield, M. J. E. Coode, & D. A. Simpson, editors. Plant diversity in Malesia III. Royal Botanic Gardens, Kew.

Vinckier, S. & E. Smets. 2002. Systematic importance of orbicule diversity in Gentianales. Grana 41(3): 158-182, 43 figs.