A Planar Essence is an item that drops from random enemy's or as a reward for completing Rifts and Invasion defeats.
There are two main categories of Planar Essences. Lesser and Greater.
Lesser Essence The basic essence usually has one stat and a resist stat. As they get higher level they will have more stats with higher values. For Example:
Bending Reed Bind on pickup Planar Essence Lesser Unique
As you can see, there are a lot more stats even on the lesser essences as you go up.
Greater Essence These are usually an "on event -> something happens" type of effect. For example, you cast a healing spell on someone, and they get an additional heal over time put on them. Or dealing damage puts a damage over time on them. Most of these types of greater essences, are classified as "Unique-Family". Unique-Family is defined by type of essence (Life/Death/Air/Water/Fire). So you can have two that say, heals over time, but they must be a different family.