On the authority of Abu 'Abd al Rahman 'Abdullah, the son of 'Umar (may Allah be pleased with them both), who said: I heard the Messenger of Allah* say:

Islam has been built on five (pillars): testifying that there is no god but Allah and that Muhammad is the Messenger of Allah, performing the prayers, paying the zakat, making the pilgrimage to the House, and fasting in Ramadan.

Obviously if one is not capable of making the trip to Hajj, it is no longer compulsory.

(Pillars is not the arabic word actually used but in this context it fits best)

Related by al Bukhari and Muslim