The block quotes are from Ms. O;'Brien:
What are the skandhas? Here is a basic guide. (The non-English names given for the skandhas are in Sanskrit unless otherwise noted.)
The First Skandha: Form (Rupa)
Rupa is form or matter; something material that can be sensed. In early Buddhist literature, rupa includes the Four Great Elements (solidity, fluidity, heat, and motion) and their derivatives. These derivatives are the first five faculties listed above (eye, ear, nose, tongue, body) and the first five corresponding objects (visible form, sound, odor, taste, tangible things).
Another way to understand rupa is to think of it as something that resists the probing of the senses. For example, an object has form if it blocks your vision -- you can't see what's on the other side of it -- or if it blocks your hand from occupying its space.
Each skandha has one or more core polarities associated with it, indeed we can say it is the field of that polarization that makes the skandhas identifiable as a numbered skandha. With the first skandha, it is the polarization of "inside and outside", "subjective and objective", that is primary.
The Second Skandha: Sensation (Vedana)
Vedana is physical or mental sensation that we experience through contact of the six faculties with the external world. In other words, it is the sensation experienced through the contact of eye with visible form, ear with sound, nose with odor, tongue with taste, body with tangible things, mind (manas) with ideas or thoughts.
It is particularly important to understand that manas -- mind -- in the skandhas is a sense organ or faculty, just like an eye or an ear. We tend to think that mind is something like a spirit or soul, but that concept is very out of place in Buddhism.
Because vedana is the experience of pleasure or pain, it conditions craving, either to acquire something pleasurable or avoid something painful.
By referring to "the external world" Ms. O'Brien is showing that she is standing on the first skandha as if it exists objectively. Here we see how the skandhas begin to construct a worldview, or a view of reality. The second skandha is also called "reception" because it acts in relation to this primal split between internal and external. In other words, what we call sensory data is discriminated on the basis that there is an internal and external reality and that the data is coming from an external reality. This works fine for light and sound which we say come from outside, and becomes a little fuzzy with smell and taste as they are sensed as being in the nose and mouth, and then very fuzzy with touch sensations in the body and completely fuzzy with ideation in the mind.
By receiving sensory data pre-screened as it were by the first skandha's polarized division into inside and outside, (me and not me, etc.) the next primary polarization is the allotment of that sense data into the categories of the primary characterization of "pleasure and pain" or "attractive and repulsive", etc. Now we have four boxes for every sensory quantum which are (1) the inside and pleasurable, (2) the inside and not pleasurable, (3) the outside and pleasurable, and (4) the outside and not pleasurable. This basic framework is the foundation for the construction of the house of views that we build.
The Third Skandha: Perception (Samjna, or in Pali, Sanna)
Samjna is the faculty that recognizes. Most of what we call thinking fits into the aggregate of samjna.
The word "samjna" means "knowledge that puts together." It is the capacity to conceptualize and recognize things by associating them with other things. For example, we recognize shoes as shoes because we associate them with our previous experience with shoes.
When we see something for the first time, we invariably flip through our mental index cards to find categories we can associate with the new object. It's a "some kind of tool with a red handle," for example, putting the new thing in the categories "tool" and "red." Or, we might associate an object with its context -- we recognize an apparatus as an exercise machine because we see it at the gym.
The Fourth Skandha: Mental Formation (Samskara, or in Pali, Sankhara)
All volitional actions, good and bad, are included in the aggregate of mental formations. How are actions "mental" formations? Remember the first lines of the dhammapada (Acharya Buddharakkhita translation)--
Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with an impure mind a person speaks or acts suffering follows him like the wheel that follows the foot of the ox.
Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with a pure mind a person speaks or acts happiness follows him like his never-departing shadow.
The aggregate of mental formations is associated with karma, because volitional acts create karma. Samskara also contains latent karma that conditions our attitudes and predilections. Biases and prejudices belong to this skandha, as do interests and attractions.
The Fifth Skandha: Consciousness (Vijnana, or in Pali, Vinnana)
Vijnana is a reaction that has one of the six faculties as its basis and one of the six corresponding phenomena as its object. For example, aural consciousness -- hearing -- has the ear as its basis and a sound as its object. Mental consciousness has the mind (manas) as its basis and an idea or thought as its object.
It is important to understand that consciousness depends on the other skandhas and does not exist independently from them. It is an awareness but not a recognition, as recognition is a function of the third skandha. This awareness is not sensation, which is the second skandha. For most of us, this is a different way to think about "consciousness."
It is also important to remember that vijnana is not "special" or "above" the other skandhas. It is not the "self." It is the action and interaction of all five skandhas that create the illusion of a self.