Stem cells are specialized cells that differentiate into various types of cells and form many organs in the human body. The chapter covers the basic and advanced knowledge of stem cells along with their application in the treatment of many kinds of diseases including, neurological disorders, gastrointestinal diseases, cancer, autoimmune disease, and osteoarthritis. Stem cells are classified into embryonic stem cells and adult stem which can be further differentiated into many types based on differentiation such as unipotent, bipotent, multipotent, and pluripotent. In the human body, blood cells such as RBC, WBC, and platelets are formed from the specific pluripotent hematopoietic stem cells (PHSC). The stem cells and their classification are elaborated on in detail. The molecular mechanism of stem cells in aging is exclusively discussed. Stem cell diversity is evident across different organs, with each exhibiting unique characteristics and often bearing distinct names. The stem cells reside in specific areas known as niches and maintain homeostasis. The role of stem cells in homeostasis is also elucidated.