Ganymede is the largest of Jupiter's moons and the largest in the entire Solar System. It is even larger than the planet Mercury, but only about half the mass. This means it must be much less dense. That said, it is still twice the mass of Earth's Moon.
It only takes 7 days for Ganymede to orbit Jupiter and is so large it has it's own magnetic field (tiny compared to Jupiter's)- but has no atmosphere around it.
Ganymede's surface is made up of a mixture of rock and water ice. The recent Galileo spacecraft found evidence for a saltwater ocean nearly 200 km below its surface, sandwiched between layers of ice. It is thought that there is more water in this ocean, than in all of the oceans on Earth. There is a small chance that simple forms of life may exist in that ocean.
The surface appears complicated with mountains, valleys, flat plains and craters. It also has many light and dark regions. The darker regions have more craters and so are thought to be older, with the light regions likely to be caused by recent plate movement.