Stem cells are undifferentiated cells capable of self-renewal and differentiation. They include totipotent (e.g., embryonic stem cells), pluripotent (e.g., induced pluripotent stem cells, iPSCs), and multipotent stem cells (e.g., hematopoietic stem cells). Their key roles involve maintaining cell populations through division and differentiating into specialized cells for tissue repair and regeneration. Embryonic stem cells, derived from early embryos, can form all cell types. Adult stem cells reside in specific tissues for local renewal, while iPSCs are reprogrammed somatic cells, circumventing ethical issues. Applications span regenerative medicine, disease modeling, and drug testing. However, challenges like low differentiation efficiency, tumorigenic risks, and ethical debates persist. Current research focuses on optimizing protocols and understanding regulatory mechanisms to unlock their full therapeutic potential.