Stem cells are distinguished by three key features: 1) self-renewal, 2) potency, and 3) plasticity. Self-renewal allows them to divide and produce identical copies of themselves over long periods, unlike regular cells that have limited division capacity. Potency refers to their ability to differentiate into various specialized cell types. Plasticity reflects their ability to adapt and transition into different cell types under specific conditions. In contrast, most other cells, like muscle or nerve cells, are highly specialized and lack both the ability to renew indefinitely and the flexibility to change into different cell types. This unique combination of features makes stem cells vital for tissue repair and regenerative medicine. 

Stem cells are formed during early embryonic development, beginning shortly after fertilization. When a sperm fertilizes an egg, it creates a single cell called a zygote. This zygote undergoes multiple rounds of cell division to form a blastocyst, a hollow ball of cells that appears around 5–6 days after fertilization. Inside the blastocyst is a group of cells called the inner cell mass (ICM) forming Germ Layers, which contains embryonic stem cells. These cells are pluripotent, meaning they have the potential to develop into any cell type in the body.

As development progresses, some stem cells become more specialized, giving rise to adult stem cells, which are found in various tissues throughout life. These adult stem cells retain the ability to self-renew but are more limited in the types of cells they can become, playing a key role in maintaining and repairing tissues in the body. 

Thus, stem cells arise early in development and are essential for growth, tissue formation, and later repair.