stem cells are cells that have not fully developed and have the ability to grow into a number of different cells. there are different types of stem cells, limited by what they can become. Some however can develop into any type of cell. Stem cells are used to try to regrow tissue that could otherwise not be regenerated such as nerves.
Cloning involves the DNA of an organism being harvested from a few cells and placed inside of an egg, in an attempt to create identical organisms. They problem with this is that the DNA from the adult has replicated many times since its youth, which causes the telomere shrink. The clone then cannot live a full life because it has a limited number of replications available. It is like taking a 30 year old and starting them as a baby... they would only have about 50 years left