There are two kinds of checkpoints, blue and purple. The former are the kind for paved roads, while the latter are for sand roads. There is no functional difference between the two. Their set ID numbers are #40 and #42 respectively.
If a checkpoint is positioned in an angled state (e.g. 45 degrees) (only possible by editing the game files), colliding with the checkpoint will send cars flying, usually at higher altitudes, mainly because the game treats it as a ramp once angled.