Question

How do stem cells differ, in structure and function, from other cells in your body?

Answer 1

Stem cells are effectively progenitor cells, from these other differentiated cells with specific properties can arise. A stem cells is a cell capable of unlimited replication and possess a higher repair ability for DNA damage than other cells in the body. These cells technically have the ability to produce all proteins within the cell whereas normal cells are limited to a set specific to their role. One protein only found in stem cells and some cancer cells is the telomerase enzyme - this extends the telomere region at the end of the chromosomes and in doing so it provides the immunity from senescence (cell death of 'old age'). This is because with each replication telomeres are normally shortened by replication eventually leading to a failure to replicate and programmed cell death but telomerase stops this shortening.
From stem cells arise all the other cells in the human body. In tissue areas there is normally a low proportion of stem cells (up to 10% total) which divide to provide new cells. To stop these all being identical to parent cells an asymmetric distribution of cellular factors at the point of division means that one of the daughter cells will be specific, such as a blood cell, while the other remains the same as the mother stem cell.

So the main differences between stem cells and normal cells in a human are; 1) The ability to self-renew and 2) Potency allowing them to produce differentiated offspring cells.

For further note is has been found that cancers also contain cancer stem cells, from these it is thought that the cancer cell population is maintained and new mutations can spread out from here but evidence is mixed so far.

Anything else there's always wiki: http://en.wikipedia.org/wiki/Stem_cell