economists point out that casual wage labourers are the most vulnerable among the three categories. could you locate who these workers are and where they are found and why?

Casual workers are those workers who work on the basis of daily wages. These workers are not permanently hired by their employers and work either on daily basis or for a specific period. Casual workers are more vulnerable than the other two categories(self-employed and hired workers). They receive wages on  a daily basis and are deprived of the benefits of the social security such as Provident Fund, Gratuity, pension, etc. 

Such workers are to be found engaged in such jobs that require low skill and education. For example, a person working on a construction site or a maintenance worker in a hotel is a casual worker. However, sometimes it may also happen that due to unemployment even the skilled and educated people in urban areas opt for working as a casual labourer.