Sterols, as they pertain to plants, are called are called phytosterols. Those of animal orgin are called zoosterols and include cholesterol and some steroid hormones. The most important phytosterols are campesterol, sitosterol, and stigmasterol.

In animals, sterols play essential roles in the physiology. For example cholesterol forms part of the cellular membrane where its presence affects the cell membrane's fluidity and serves as secondary messenger in developmental signaling. In humans sterols act to provide important signals and metabolic communications eg. circadian rhythms, blood clotting.

Plant sterols are also known to block cholesterol absorption sites in the human intestine thus helping to reduce cholesterol in humans.

References

Fahy E, Subramaniam S, Brown HA, et al (2005). "A comprehensive classification system for lipids". J. Lipid Res. 46 (5): 839–61. PMID 15722563.?