What is the difference between a stellar mass black hole and a super massive black hole?
In terms of physical properties, the difference between stellar mass black holes is their mass: stellar mass black holes are around 3-10 times the mass of the Sun, whilst supermassive black holes are 105-1010 times the mass of Sun. Supermassive black holes are just bigger versions of stellar mass black holes, but behave in the same way (just scaled up).
There are other differences, which are related to their formation. Stellar mass black holes form from the collapse of massive stars at the end of their lives. You can then find them scattered throughout galaxies, just like you find massive stars.
Supermassive black holes are found at the centres of galaxies. We are not exactly sure how they form, although we do have a number of ideas. They are too big to have formed from a collapsing star. We believe that quasars are powered by matter accreting onto supermassive black holes, and measurements of these show that these can grow to a billion solar masses in less than a billion years from the big bang. This means will need a highly efficient way for them to gain mass. We also observe that the properties of the surrounding galaxies are correlated to their central black hole's mass (this is most famously known as the M-σ relationship in astronomy, as we use M for the black hole mass and the Greek letter sigma for the velocity dispersion, a speed characterising how fast stars are moving). This correlation indicates that the growth of the black hole and galaxy are probably linked somehow.