A citta dhamma arises and then instantly disappears again. Another citta arises and disappears. The next arises and disappears. Citta does not arise alone. Cetasikas arise and disappear along with citta. When cittas arise, they are always accompanied by cetasikas. Indeed, consciousness cannot exist alone in the absence of mental factors and material phenomena. Citta itself has only one quality — the ability to cognize an object. Cetasikas help the citta with details in its act of cognition, and rupas provide the stage.
Listed below are the various groups of dhammas. The numbers show how many individual dhamma types there are in each subgroup, which together make up 89 (121) cittas, 52 cetasikas and 28 rupas (figures in parentheses apply to advanced meditation states).
The brief moment when mental factors (cetasikas) and material phenomena (rupas) arise, exist and disappear together with a discrete package of consciousness (citta) is called a mind moment. Each such mind moment manifests as one of the 89 (121) different types of consciousness. The ordinary mind experiences the unfolding of the mind moments as one continuous stream of consciousness, but a mind that has attained advanced stages of insight meditation (vipassana) experiences consciousness as a series of discrete entities. The mind capable of attaining even the highest stages of vipassana meditation is able to identify the characteristics of individual moments of awareness, as well as their causal relationships to other dhammas and mind moments. This, in brief, is how the knowledge found in the Abhidhamma — most prominently in its Patthana part — came about.
The names of the various groups and subgroups above may suggest some of their characteristics. Further details can be found elsewhere on this site, which is dedicated to presenting the details of the knowledge contained in the Patthana book in a non-academic manner.