The Abhidharmasamuccaya First part:
The Akranasamuccaya: The Collection of Characteristics

Chapter 1
The first section details the enumeration, characteristics and definitions of the « three dharmas ».

The Three Dharmas are:
1.The five aggregates (skt: skandha; tib: phung po)
The five aggregates - form, sensation, perception, formations, and consciousness - represent the ways in which the self appears, namely, self as the physical body; self as the basic sensations of pain and pleasure; self as identification with perception ; self as the constructor of all karmic formations, and self as the conscience in which all these are fixed.

2.The eighteen elements (skt: dhatu; tib: khams)
  These are the six sense faculties, their six objects, and their six corresponding consciousnesses. Based on the elements which apprehend the objects, the functioning of the skandhas maintains the experience of a self in the present and the past. The elements are the base out of which all phenomena arises. Therefore, the elements are the seed of the functioning of cause and effect (karma).

For example:
« What characterizes the element of the visual consciousness? It is a reaction concerning a visible form, which has the eye as its subject and the form as its object, and as a result a seed accumulated in the fundamental consciousness (alaya). »
Asanga: The Anthology of Special Topics of Knowledge, translated and commented by Walpola Rahula

3.The twelve spheres (skt: ayatana; tib: skye mched bcu gnyis)
The twelve spheres are the twelve sources of cognition: the inner six organs that apprehend (eye,ear,nose, tongue,skin, mind) and the outer six objets which are apprehended (form,sound, odour, taste, touch, mental object). They include all conditioned and unconditioned phenomena.

The sense objects, the sense organs, and the associated consciousnesses are the source, or the access door to future experience.

« They are the necessary medium for giving birth to and developing dualistic knowledge in terms of subject and object. »
Cornu Philippe: Dictionnaire encyclopédique du bouddhisme

« Thus the Bouddha said: Matter is like foam, sensation like a bubble, perception like a mirage, formations like a banana tree, and conscience like an illusion.»

« The meaning of these words is the absence of self, of impurity, of a lack of satisfaction, an absence of solidity and of substantialness.»
Asanga: The Anthology of Special Topics of Knowledge, translated and commented by Walpola Rahula

The second section is named « The division of aspects »
Following this habitual structure of the Abhidharma, Asanga adds a second section, « the division of aspects (or associations) », which again takes up the three dharmas, discussing them under several different aspects, such as their substance, their designation, their relative and ultimate aspects.

This section seeks to establish the absence of self (skt:atman; tib:bdag) of all phenomena.
Through analyses according to the division of aspects, the student understands the defective nature of mental obscurations associated with the notion of a self. By meditating on the aggregates, the elements and the ayatanas as well as the erroneous behaviour it ensues, the student can abandon the very cause of suffering.

« What is the absence of illusion? It is the knowledge of the result of actions... Its function is to provide a foundation to the stopping of bad actions. »
Asanga: The Anthology of Special Topics of Knowledge, translated and commented by Walpola Rahula

Chapters 2 3 and 4

The second, third and four chapters give a synthetic view over the “three dharmas”, which represent the entire range of phenomena. They are structured and analysed in such a way that the student understands the interaction of aggregates, elements and spheres (skandha, dhatu and ayatana) and integrates the reality of phenomena as being “mind-only” (skt: cittamatra; tib: sems tsam). It shows the falsity of belief in a self which experiences, perceives, desires, and remembers.

Moreover, the student is led to understand what to cultivate and what to give up to attain perfect buddhahood.

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