An Introduction to the Aggregates
The historical Buddha spoke often of the Five Skandhas, also called the Five Aggregates or the Five Heaps. The skandhas, very roughly, might be thought of as components that come together to make an individual. Every thing that we think of as "I" is a function of the skandhas. Put another way, we might think of an individual as a process of the skandhas.
When the Buddha taught the Four Noble Truths, he began with the first Truth, life is "dukkha." This is often translated as "life is suffering," or "stressful" or "unsatisfactory." But the Buddha also used the word to mean "impermanent" and "conditioned." To be conditioned is to be dependent on or affected by something else. The Buddha taught that the skandhas were dukkha.
The component parts of the skandhas work together so seamlessly that they create the sense of a single self, or an "I." But the Buddha taught that there is no "self" occupying the skandhas. Understanding the skandhas is helpful to seeing through the illusion of self.
Please note that the explanation here is very basic. The various schools of Buddhism understand the skandhas somewhat differently, so as you learn more about them you may find that the teachings of one school don't exactly match the teachings of another. The explanation that follows is as nonsectarian as I could make it.
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. 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. The Third Skandha: Perception (Samjna, or in Pali, Sanna)
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 aggragate of mental formations. How are actions "mental" formations? Remember the first lines of the dhammapada (Acharya Buddharakkhita translation)--
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.