Dukkha: Refers to the REALITY of suffering. A Pali word, it means 'pain', 'suffering', 'sorrow' or 'misery' as well as 'imperfection', impermanence', emptiness' or 'insubstantiality'.
There are three kinds of suffering:
Ordinary Suffering - dukkha-dukkha
There are all kinds of suffering in life: birth, old age, sickness, death, association with unpleasant persons and conditions, separation from beloved ones and pleasant conditions, not getting what one desires, grief, lamentation, distress. All these are forms of physical and mental suffering.
Suffering produced by Change - virapinama-dukkha
Pleasant and happy feelings or conditions in life are not permanent. Sooner or later they change. When they change they may produce pain, suffering, unhappiness or dissappointment.
Suffering as Conditioned States - samkara-dukkha
An 'individual', an 'I' or a 'self' is a combination of ever-changing mental and physical forces which can be divided into five groups or 'aggregates' pancakkhandha. Suffering as conditioned states is produced by attachment to the Five Aggregates.