Acid Blade

To clarify:

This ability applies a Preparation poison to you as a buff that lasts for 15 seconds and doesn't do anything beneficial until you use Hidden Strike or Backstab. Once you use Hidden Strike or Backstab on a target this buff becomes a second buff that lasts for 15 seconds and gives you (currently now) 30% armor pen (meaning Kinetic and Energy damaging attacks YOU deal bypass 30% of the targets armor worth of damage reduction aka 30% of a targets armor becomes worthless).
In addition to the armor pen bonus to you it applies a DoT (Damage over Time) component to the target acting as a poison effect (for the purpose of Tactical Opportunity and Culling) that deals the listed amount of damage as internal damage (aka bleed damage) divided over 6 seconds.

The DoT bypasses armor and is only reduced by internal damage resistance that is usually only supplied by tanks or Consular/Inquisitor buffs (10% resistance to elemental and internal damage)

It also actually does costs 15 energy. Putting a point into Waylay can reduce the cost by 2 energy.

Its hardly completely incorrect because it does exactly what it says it does but it was reworded recently to help out those who were confused. It DOES a damage component though it is not direct damage it is DoT damage, exactly like it says. It also does Internal damage exactly as it says. It also provides Armor Pen just like it says only it doesn't give armor pen UNTIL the strike is made which was slightly reworded so people don't think that Hidden Strike gains the armor pen bonus... because it doesn't. The armor pen is applied AFTER the initial application of Acid Blade to a target, however the armor pen is not applied TO the target it is a buff applied to the player for 15 seconds. The Damage Over Time component is applied for 6 seconds to the initial target of the hidden strike or backstab. You CAN however use acid blade, hidden strike or backstab a target, cloak screen and use hidden strike or backstab again and still get the armor pen bonus as long as its within the 15 second window.

A correction to my above post and an addition to mort's -

This ability grants you a short buff that causes your next backstab or hidden strike to proc acid blade on the target, a 6 second poison effect that deals a good amount of damage, and counts towards both Tactical Opportunity and Culling. In addition, after the ability procs, it gives you a buff called Acid Blade that grants 50% armor penetration.