Mars has a composition very similar to Earth's. The surface is primarily made from Iron (III) Oxide which on Earth we usually call "rust". The rocks on the surface are otherwise quite similat to the types found on Earth. Most rocks are made from metal bonded to non metals like carbon, silicon, phosphorus and oxygen and this is exactly what we find on Mars. Mars has crystals the same we way do and it also has deposits of pure metals like Iron, much the same as Earth.
Where MArs is different to Earth is in the atmosphere and at the poles. Whereas our air is mostly Nitrogen with some oxygen and less than 1% Carbon dioxide (with the polar caps made of water), Mars' atmosphere is made of around 95% Carbon dioxide with some Nitrogen thrown in (the polar ice caps being made of solid Carbon dioxide).
We have also, of course, recently discovered liquid water on the surface of the planet, which means that Martian soil could support bacterial or even plant life similar to Earth's, but the air would make it unbreathable for Earth animals. The fact that there's virtually no oxygen in the air suggests there aren't any plants undergoing photosynthesis there however, so if we were to find cellular life there it would be very different in behaviour and chemistry to Earth life.