Graves are the remains of hedgehogs which have died. They are created as objects when hedgehogs die on land because its health got down to 0.
There are no graves for hedgehogs which have drowned or exploded in a kamikaze.

Graves as such are not very interesting, they just stand around and fall straight down when land is removed below them.

But graves can be used by a resurrector to bring dead hedgehogs back to life.

Each team can have their own grave style.

• Animated preview of graves

The HWKB has an article on creating your own custom graves.