Sightings was rolled out to everyone, and is not as good as the original tracker but at least it's accurate (which we can't say about the tracker in effect for the last several weeks). Sightings basically tells you all the pokemon currently very close to you but not much else. However, if a pokemon drops from the list then you at least know it is in the opposite direction you are walking, and pokemon on the list are not too far away. It doesn't tell you anything about the relative distance of what is on the list, though.
The new Nearby feature is different and is what WK described above. It's only in effect in certain areas.