Concealment: Your target has concealment when he is, for whatever reason, hard to see but not totally invisible. Cover or concealment is necessary for a character to make a Stealth check. Even if the character isnét hiding, concealed creatures add +1 to Target. Example: smoke, fog, tall grass, an Invisible creature covered in dust or flour, darkness without lowlight vision.
Lighting: For game purposes, lighting comes in three types: clear, hazy, and total darkness. In clear light, characters operate without penalty. In hazy light, everything has concealment. In total darkness, everything is invisible.
Invisibility: When a creature or object cannot be seen, it is invisible. Invisibility is relative ä a hiding character who made a DC 6 stealth roll is invisible to characters who rolled 5 or lower on perception, but visible to those who rolled 6 or higher. If you try to attack an invisible character, you must first locate its square ä there are a few ways to do this, (guessing, having it pointed out, etc.) but the most common way is simply by hearing the target. Invisible creatures get +4 Target.