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ELI5: What are stem cells and for what can they be used

Hi, i was just wondering what are stem cells and why are they so important...

First post :D

Sorry for my english.
gpt
Stem cells are undifferentiated cells.   
What that means is that they haven't yet decided what they are, so we can tell them to be whatever we want them to be. If we want to look at lung cells we tell the cells to become lung cells. If we want to look at kidney cells, we can tell them to become that.    
    
They can be used in a lot of applications, but what's being investigated most heavily is curing things that the body can't heal itself.   
Neurological disorders like Parkinson's and Alzheimer's have no natural repair function, but stem cells can be told to turn into new neurons.    
Another example is that they can be told to turn into insulin producing cells to cure diabetes.    
   
So they have a really great potential for curing diseases that have no other treatments.