What exactly is a hormone?

BIG question!: Hi. In general terms, a hormone is a substance made in one place in the body and active at other places (the target tissues). Hormones bind specific receptors in target tissues and modify cell/tissue function. Hormones come in 4 broad categories: peptides/proteins, steroids, biogenic amines, and fatty acid metabolites. I've got monster textbooks that try to answer that question! :-)