An auxin is any of several
hormones that play a role in many aspects of
plant growth and
development. Auxins include indoleacetic acid, phenylacetic acid, and 4-chloro-indoleacetic acid.
Commercially, auxins are used to promote root growth, to promote uniform flowering, and to set fruit and prevent premature fruit drop. Synthetic auxins such as 2,4-D and 2,4,5-T have been used as herbicides; broad-leaved weeds like dandelions are much more susceptible to auxins than narrow-leaved plants like grass and cereal crops.
The defoliant Agent Orange was a mix of 2,4-D and 2,4,5-T. 2,4-D is still in use and is thought to be safe, but 2,4,5-T was more or less banned by the EPA in 1979. The dioxin TCDD is an unavoidable contaminant produced in the manufacture of 2,4,5-T; as a result of the the integral dioxin contamination, 2,4,5-T has been implicated in leukemia, miscarriages, birth defects, liver damage, and other diseases.
Some of this writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/