It depends on many things - the age of the person, other diseases he or she may have, whether he experienced any trauma to the joint, drugs or other substances he or she uses. Generally speaking, it may be caused by an infection (usually after trauma, very painful and abrupt inflammation, necessitates rapid treatment), rheumatologic diseases (such as rheumatoid arthritis or others, reaction to drugs or as a feature of other diseases.
You may read more here:
http://en.wikipedia.org/wiki/Arthritis 