Tools for creating academic models

Hi,

I am doing a Ph.D. research and always using my Mac to write my research. In addition to writing text, I sometimes need to create models that represent my research findings. Microsoft word doesn't help me to do that since I need to something professional that is easy to understand shows all relationships among different components. Are there any recommended tools which are simple that allow me to create models? I mean tools that can help me draw circle, squares, arrows, etc with flexible functionalities.

Thanks