The first four jhanas are “form” absorptions, marked progressively by decreasing effort and increasing refinement: rapture and pleasure yielding to equanimity. The four “formless” absorptions extend further into boundlessness of space, boundlessness of consciousness, nothingness, and neither-perception-nor-non-perception.
Jhana is cultivated through Shamatha — sustained concentration on a single object, often the breath — until the mind unifies and shifts into absorption. The jhanas are not the goal; traditionally they are used as platforms from which Vipassana produces liberating insight.
Contemporary Theravada teachers differ on how deeply traditional jhana is accessible to modern practitioners. The topic is one of the live debates in 21st-century Buddhism.