Hydrogels are a unique class of three dimensional cross-linked polymeric networks that can hold a large fraction of aqueous solvents and biological fluids within their structures. Nowadays, hydrogelshave attracted a growing interest of many scientists in different fields of research. Intelligent hydrogels have found a significant role in a wide variety of applications such as drug deliverysystems, tissue engineering, optics, diagnostics and imaging. The purpose of this paper is to present a brief review on the basis concept of hydrogels, the description about classification, synthesismethods, stimulation situations, relevant mechanisms, and applications.