There are many status effects out there, each of them harmful. They can alter a being's capabilities in a variety of ways and often come about because of special abilities, such as spells or ESP. While status effects rarely if ever lead to direct damage, they linger and harm the one suffering from them in much more insidious ways. In that sense, they are just as important to keep in mind and make use of as direct damage.
A status effect lasts until its duration runs out (depending on its source) or until it is healed with an ability that is powerful enough to do so. If multiple status effects fall upon a being at the same time, they do not mix. Each has its own duration and own effect, and they will not interact with each other.
Here is a list of basic status effects and what they do. If come up with your own effects, you can suggest them as part of your abilities.
Blinded
Charmed
Deafened
Fatigue
A special condition that gains levels, which bring additional problems to an already bothersome status effect.
Frightened
Incapacitated
You are at the death's door thanks to the wounds suffered in a battle, and do not have long to live anymore.
Invisible
You disappear from sight and cannot be visually seen.
Paralyzed
Petrified
Poisoned
Prone
Restrained
Stunned
Unconscious
You fall unconscious and become unable to act thanks to your dazed out mind.