Ecdysteroids are a class of compounds (polyhydroxylated ketosteroids, with various tails) that are structurally similar to androgens. They are well studied as plant and insect growth factors, and derived their name (ecdy-) from the process of molting in insects, called ecdysis.
Ecdysteroid is a category, and popular ecdysteroids include 'ecdysone', 'ecdysterone', 'turkesterone' and '20-hydroxyecdysone'. These four are the most commonly studied, but each ecdysteroid shares the same general properties although varies in potency and effects slightly. Turkesterone appears to be the most anabolic.
They have some biological effects in mammals when orally ingested, and have been called by some researchers as "behaving similar to anabolic steroids putatively without the androgenic effect". Due to the lack of androgenicity, their safety profiles are much greater than anabolic androgenic steroids.
Additionally, they seem to have a wide variety of side-effects that are deemed as healthy. Ecdysteroids can lower cholesterol and blood glucose, are seen as healthy for the liver and intestines by increasing protein synthesis rates, and may have protective effects on neural tissue.
There is a lack of trials currently available for humans, but promising evidence is available for in vitro studies on human muscle fibers as well as a multitude of animal models showing enhanced growth rates with ecdysteroid ingestion.