This document discusses hydrogels, which are 3D polymer networks that can absorb large amounts of water while maintaining their shape. It provides a brief history of hydrogels and classifications based on generation. Stimuli-responsive or "smart" hydrogels that change properties in response to environmental stimuli are highlighted. Characterization techniques and applications of hydrogels in biomedical areas like drug delivery, cell encapsulation, and tissue engineering scaffolds are summarized.