What are stem cells?
Stem cells are fundamental components of every tissue and organ in your body. There are different types of stem cells, and they can originate from various places in the body; they are also formed at different times in our lives. Some of “these include embryonic stem cells that exist only at the earliest stages of development and various types of tissue-specific (or adult) stem cells that appear during fetal development and remain in our bodies throughout life.”
Why are they important?
Stem cells have unique properties that make them extremely important. Specifically, stem cells hold the “remarkable potential to develop into many different cell types in the body during early life and growth. In addition, in many tissues they serve as a sort of internal repair system, dividing essentially without limit to replenish other cells.” What this means is that “stem cells regularly divide to repair and replace worn out or damaged tissues,” thus working to maintain health and rejuvenation within the human body.
While stem cells are by nature unspecialized, they are also important because they can “give rise to specialized cells” via a process known as differentiation.  As such, a stem cell may start out in a universal context, but can go “through several stages, becoming more specialized at each step,” thus taking on a more specific role in the body over time.
Stem Cells and Cancer Treatment
Stem cells are found throughout the body, but when used to treat cancer, there are 3 key sources  within the body. They are:
Bone marrow, from you or someone else
The bloodstream, from you or someone else
Umbilical cord blood from newborns
Stem cells are used for regenerative purposes in a treatment known as engraftment. During this process,stem cells are used via transplant, alone or in conjunction with other treatments, to “try to kill all the cancer cells. This treatment also kills the stem cells in the bone marrow. Soon after treatment, stem cells are given to replace those that were destroyed.” Stem cells are injected into the body intravenously, in a similar fashion to that of a “blood transfusion. Over time they settle in the bone marrow and begin to grow and make healthy blood cells.”
Future Potential of Stem Cells
As researchers continue to explore the properties of stem cells, major breakthroughs have the potential to occur. For example, stem cells “may soon become the basis for treating diseases such as Parkinson's disease, diabetes, heart failure, cerebral palsy, heart disease and host of other chronic ailments.” 
For More Information
The following video was taken from a TED-Ed lecture by Craig A. Kohn and features animation explaining what stem cells are and how powerful they can be in aiding the body’s healing. For more information and resources provided during his lecture, click here.