A Log Pose is what travelers in the Grand Line use to navigate through the islands. Compasses don't work because each island has its own magnetic field. The needle on the log pose points to the next island in the Grand Line and is the only thing that travelers can rely on. When on an island, the Log Pose has to memorize the magnetic waves from that island before it can point to the next destination. Unfortunatly, this can take years but it commonly only takes days.
An Eternal Pose is like a Log Pose, except it only points to one island. No matter where you take it, it will only point to that island forever (hence the eternal) so travelers don't have to wait for it to memorize the magnetism of the island. The Strawhat Pirates have used an Eternal Pose pointing to Alabasta and Jaya.