A stem cell is a unique type of cell that has the ability to develop into many different cell types in the body. They are essential for growth, development, and tissue repair. There are two main types of stem cells: embryonic stem cells, which can become any cell type, and adult stem cells, which are more limited in their potential but play a crucial role in maintaining and repairing tissues. Researchers study stem cells to understand how they work and how they can be used in medicine. Potential applications include treating diseases like diabetes and Parkinson's disease, as well as repairing damaged tissues after injuries.