The suffering in which Siddhartha was referring to is known in Buddhism as Dukkha which has three separate classifications: first is the Dukkha-dukkha which is also referred as ordinary suffering and is attributed to birth, illness, aging and death. The second classification is Vipariṇāma-dukkha or suffering due to change, this is the anxiety that is caused by trying too hard to hold on to things which are in constant change. The third and final classification is Saṃkhāra-dukkha this is suffering which comes from feeling an overall lack of satisfaction and feeling that no what we try life never measures up to our expectations. Dukkha prevents the achievement of enlightenment or Nirvana and thus breaking the cycle of rebirth.
