It is a huge collection of systematically arranged, tabulated and classified doctrines of the Buddha, representing the quintessence of his Teaching. It is unique in its abstruseness, analytical approach, immensity of scope and conduciveness to one’s liberation.

In Abhidhamma, dhammas are no longer taught in a conventional terms ( sammuti sacca ) making references to persons and objects as ‘I’,’ we’, ‘he’, ‘she’, ‘man’, ‘dog’, ‘tree’ etc. Here the dhammas are treated entirely in terms of its ultimate reality (paramattha sacca). 

Analysis of phenomenon are to its ultimate constituents and all relative concepts such as man, mountain, etc. are precisely defined, classified and systematically arranged.

Thus in Abhidhamma, everything is expressed in terms of khandhas, five (5) aggregates of existence; ayatanas, five sensory organs and mind, and their respective sense objects; dhatu, elements; indriya, faculties; sacca, fundamental truths; and so on. 

Relative conceptual objects such as man, woman, etc. are resolved into ultimate components of khandhas, ayatanas, etc. and viewed as an interpersonal psycho-physical phenomenon, which is conditioned by various factors and is impermanent (anicca), unsatisfactoriness (dukkha) and is without a permanent entity (anatta).

The Abhidhamma approach is more thorough, more penetrating, breaking down each corporeal or mental component into the ultimate, the most infinitesimal unit.

e.g.

corporeal ( matter ) aggregate has been analyzed into 28 constituents; the aggregate of sensation into five; perception into six; the mental formation into 50 and the aggregate of consciousness into 89.

Hence, the Abhidhamma Pitaka is made up of seven (7) massive treatises.

A. Dhammasangani Pali (Classification of Dhamma)
Containing detailed enumeration of all phenomenon with an analysis of consciousness ( citta ) and its concomitant mental factors (cetasikas).

It begins with a complete list of heads called Matika which serve as a classified table of mental constituents treated in the entire system of the Abhidhamma.

The matika consists altogether 122 groups of which 22 are called Tikas or triads, those that are divided under three heads, and 100 called Dukas or dyads, those that are divided under two heads.
 

Based on these Matikas of Tikas and Dukas, the Dhammasangani is divided into four Divisions.

B. Vibhanga Pali (Division)
Book of Analysis - whereas the 1st book provides a bird’s eye view with systematic arrangements, Vibhanga give a closer view of selected portions of those groups bringing out minute details. 

Thus Dhammasangani explains what and how many khandha, ayatana etc., Vibhanga provides full knowledge concerning them, stating the exact nature of each dhamma, its constituents and its relationship to other dhammas.

Vibhanga then is divided 18 chapters each dealing with a particular aspect of the dhamma; its full analysis and investigation into each constituents.

C. Dhatukatha Pali (Discourse of Elements)
A separate treatise devoted to the thorough analysis of dhatus ( matters ). 

Dhatukatha studies how the dhammas listed in the Tikas and Dukas are related to the three categories of khandha, ayatana and dhatu in their complete distribution i.e. five khandhas, twelve ayatanas and eighteen dhatus.

These are discussed in 14 ways of analytical investigation which constitute the fourteen chapters of Dhatukatha.

D. Puggalapannati Pali (the Book of Individuals)
A small treatise giving a description of various parts of individuals according to the stage of their achievement along the Path. Different types of individuals are classified, in ten chapters of the book, after the manner of enumeration employed in Anguttara Nikaya.

E. Kathavatthu Pali (Points of Controversy)
It is a compilation by the Venerable Moggaliputta, the presiding thera of the 3rd Great Council in which he discusses and refutes doctrines of other schools of thought in order to uproot all the points of controversy on the Buddha-dhamma.

It does not directly deal with the abstruse nature of the dhamma. 
It is mainly concerned with wrong views such as 'person exists; self exists' and 'arahat falls away from arahantship.'

The style of compilation of this treatise is quite different from that of others, written as it is in the form of dialogue between two imaginary debaters, one holding the heterodox views of different sects and the other representing the orthodox views.

F. Yamaka Pali (the Book of Pairs)
Yamaka sets out to define and analyse the interrelationship of dhammas and puggalas ( individuals ) as they exist together in these three worlds. This is accomplished in the form of pairs of questions. 

Yamaka is regarded as a treatise on applied logic in which analytical procedure is arranged in pair.

The logical process of conversion and complete inversion is applied to determine the complete import and limit of a term in its relationship with the others. An equivocal ( same sound ) nature of a term is avoided by showing, through such arrangement of questions, how other meanings of the term do not fit for a particular consideration.

e.g.

‘ May all rupa be called rupakkhandha ?’ 
  No, rupa also used in expression like piya-rupa ( loveable nature ) etc. there does not mean rupakkhandha.
‘ May all rupakkhandha be called rupa ?’ 
  Yes, rupakkhandha is a very wide term.

The arrangement of the system is so very intricate, complex, highly thorough and complete that it earns for this treatise the reputation of being deep, profound and unfathomable.

It arranges all conditioned things ( 22 tika and 100 duka ) under twenty-four kinds of relations, describes and classifies them into a complete system for understanding the mechanics of the universe of dhamma. 

The whole work is divided into four great divisions;

a. the studies of instances in which paccaya relations do exist between the dhammas.
b. the studies of instances in which paccaya relations do not exist between the dhammas.
c. the studies of instances in which some of the paccaya relations do exist between the dhammas
    but the others do not.
d. the studies of instances in which some of the paccaya relations not do exist between the dhammas
    but others do exist.

The 24 paccaya relations are applied to the four great divisions in six ways ;
a. in their 22 tika group
b. in their 100 duka group
c. in their 100 duka mixed with 22 tika groups
d. in their 22 tika mixed with 100 duka groups
e. in their 22 tika group mixed with one another
f. in their 100 duka group mixed with one another