i was taught in a social work subject that you are homeless if you don't have a permanent address for example if you are couch surfing, living in a shelter, living on the streets etc.  some people find themselves homeless due to circumstances, other choose to be so (they may consider being on the streets safer that in a house after experience of abuse).  

there are many reasons why people become homeless and it does not take much of a change in circumstances for some one to find themselves in such a position, most often circumstances out of the persons control.