Those of us who were subjected to sleepaway camp at an early age may already be familiar with this feeling - for the rest of you lucky mortals who missed out, homesickness is essentially an acute form of anxiety or emotional distress that results from feeling disconnected from familiar people and places and forced out of your regular routine.
Homesickness can arise from a number of different factors -- difficulty adjusting to a new environment, feeling lonely or cut off from your regular support system, confusion or problems understanding a new environment/culture/language, a perceived lack of control over what's happening around you, culture shock, and the list goes on.