To know clearly about the population What is the difference among target population,study population and theoretical population ?
 A: They can be confusing. However attempts were made to define them:

Two Types of Population in Research
Target Population
Target population refers to the ENTIRE group of individuals or objects
  to which researchers are interested in generalizing the conclusions.
  The target population usually has varying characteristics and it is
  also known as the theoretical population.
Accessible Population
The accessible population is the population in research to which the
  researchers can apply their conclusions. This population is a subset
  of the target population and is also known as the study population. It
  is from the accessible population that researchers draw their samples.

Here is another example: 



Basically, target population (also known as theoretical population) is the group to whom we wish to generalize our findings.
Study population (also known as accessible population) is the actual sampling frame, from which we randomly drew our sample. This is the group to whom we actually can generalize our findings, because we sampled from them.
Having said these, for safety I would recommend checking the glossary and definition of the text to make sure the authors really do use the same system.
A: Easily speaking:
Target population is the population you are interested in your study; 
Study population is a sub population that you are taking from the target population for doing your study
Theoretical population is the same as target population, which is the population you want your study to be generalized to. 
For example, you want to do a study: the average height of all men between age 20 to 30 in Canada, then the target and theoretical population is all men between age 20 to 30; and your study population would be say all men between age 20 to 30 in Ontario.
A: Target population, as the name suggests, is the population that is of interest to the researcher. Also known as the theoretical population, it serves as the main environment for the researcher's hypothesis in a general term.
 For instance, one may be thinking about doing a research on the criteria for recruiting high school teachers in Liberia. High school teachers in Liberia is the target population, which is quite large, difficult to manage, difficult to sample, etc.
On the other hand, the study population, which is also known as the accessible population, is the population that is derived from the target population for the smooth conduction of the research in a specific term.
 For instance, instead of the entire population of high school teachers in Liberia, the researcher could consider doing a study on the high school teachers of the Monrovia Consolidated School System (MCSS). This specific group of people is the study population, which is manageable, much easy to sample, etc.
