Question

What are stem cells?

Answer 1

There is no scientific information as to how long a line of stem cells may be retained because stem cell banking is a relatively new phenomena (around $25$ years). However, experts believe that cryogenically maintained cells have no expiration dates, thus your stem cells may theoretically be stored for generations by combining freezing temperatures with proper cultivation.
In nature, embryonic stem cells (ES cells or ESCs) are pluripotent. They're made from the blastocyst's inner cell mass. Around $4-5$ days after fertilisation, human embryos reach the blastocyst stage, which has nearly $50-150$ cells. A blastocyst is a pre-implantation embryo in its early stages.

Complete answer:
“Stem cells are unique human cells that can differentiate into a variety of cell types, ranging from muscle cells to brain cells.”
In addition, stem cells have the ability to repair damaged cells. These cells have a great deal of healing potential. They have the ability to evolve into any sort of cell.
Types of stem cells-
1. Embryonic Stem Cells
2. Adult Stem Cells
3. Induced Pluripotent Stem Cells
4. Mesenchymal stem cells
Applications of stem cells-
Tissue regeneration- A specific type of tissue or organ can be grown using stem cells. This may be beneficial in the case of kidney and liver transplants. The stem cells from beneath the epidermis have already been used to create skin tissue that can be used to treat serious burns or other injuries by tissue grafting.
Treatment of cardiovascular disease- Using human stem cells, a team of researchers produced blood vessels in mice. The blood vessels developed their network and were as efficient as normal vessels within two weeks of implantation.
Treatment of brain diseases- Stem cells can also be used to treat disorders like Parkinson's and Alzheimer's. These can aid in the regeneration of brain cells that have been injured.
Blood disease treatment- Cancer, sickle cell anaemia, and other immunodeficiency illnesses are treated by adult hematopoietic stem cells. These stem cells can be employed to make red and white blood cells in the human body.

Note:
Sources of stem cells:
Stem cells come from several regions of the body. Adult stem cells can be found in a variety of tissues throughout the body. Mature cells are specialised to perform a variety of tasks. These cells can usually develop into the types of cells prevalent in the tissues where they reside.
Embryonic Stem Cells are pluripotent stem cells derived from $5$-day old blastocysts that develop into embryos. These cells have the ability to develop into any form of cell or tissue in the body. These cells have the ability to regenerate all of the cells and tissues that have been lost due to damage or sickness of any sort.