Textures

#1 1)Normal = sometimes called bump, gives faked levels of height on to a 2D surface. It's about how directional light info strikes a surface. Used in connection with Specular. Stops a texutre from looking flat.
2)Specular = shininess of texture, how much it reflects light
3)Ambient Occlusion = kind of a natural shadowing technique, occurs where faces meet at angles on a model.
4)Displacement = I think this one is for the "edging" of parallax mapping. Actual 3D information in a 2D texture.

edit:

Normal and Specular mapping being picked out from the direction of a light source (left) and no light source to affect Normal and Specular (right) - same as not having Normal or Specular mapping. Notice the flatness of the textures.

#2 Texture Descriptions

#3 Thanks Steve and Michael!