Condition

Certain effects apply a condition to its target(s). Conditions are usually temporary, harmful and need certain actions to be taken to get rid of faster.

If more than one condition affects a character, they are all applied. Certain effects can’t be combined, thus the most severe effect takes effect.

Conditions are not magical effects by themselves but they can be bestowed by a magical effects, like spells and weapon effects.

Complete list of conditions found in DDO.

Removing
Although death is a condition itself, it removes most of other conditions.

Usually removing the source that applies the condition removes also the condition but not always. e.g. Removing the disease blinding sickness does not remove the condition blinded it might cause.

More gently ways to remove (some) conditions are restoration, heal and similar effects. Resting and simply waiting for the duration to wear off also works.

Only condition that can be removed without magical aid or by resting is invisibility.

PnP Dungeons and Dragons
Unlike in DDO, conditions in PnP D&D are permanent. Only if something specially states its removes a certain condition, only then the condition is removed. Some conditions are missing from DDO, like confused, grappling, staggered and flat-footed to name few.

Some are working only partially, like frightened, stunned and energy drained to name few.

Some conditions are combined, like all states of fear instead of Cowering, Frightened, Shaken