Mini-Bosses (or sub-bosses) are tough mobs that are usually only found in instances on the way to an instance Boss or guarding them; most sub-areas in an instance will usually have an intermediate encounter with a "Mini-Boss". Mini-Bosses can be tougher than an elite with the same level, but may often be equivalent to the same level elite mob. Some Mini-Boss mobs are also immune to Crowd Control and may have other select immunities as well.
Some quest chains end with needing to kill what amounts to a Mini-Boss, but might also be considered a Boss so the distinction is fuzzy.
Examples of minibosses include Gatewatcher Gyro-Kill and Gatewatcher Iron-Hand in Mechanar.