Kerria is native to eastern Asia (China, Japan) and only counts a single species, K. japonica (L.) DC. It is widely cultivated as an ornamental shrub and sometimes seen in the wild in Belgium.
Kerria belongs to the tribe Kerrieae Focke, along with three other genera (Potter & al. 2007). Rhodotypos Siebold & Zucc., also grown as an ornamental shrub, is one of these and both genera are indeed similar in appearance and may be confused. Rhodotypos, however, is easily separated by its opposite leaves (alternate leaf arrangement in Kerria).
Potter D., Eriksson T., Evans R.C., Oh S., Smedmark J.E.E., Morgan D.R., Kerr M., Robertson K.R., Arsenault M., Dickinson T.A. & Campbell C.S. (2007) Phylogeny and classification of Rosaceae. Pl. Syst. Evol. 266: 5-43. [available online at: http://biology.umaine.edu/Amelanchier/Rosaceae_2007.pdf]
Wadleigh J.S. (1975) The worthy Kerrias. Arnoldia 35(3): 160-161. [available online at: http://arnoldia.arboretum.harvard.edu/pdf/articles/1975-35-3-the-worthy-...