What would be a good way to organize the field of machine learning into sub-fields, for a new research candidate to gain a good overview of the domain. Basically what are the different streams of research active currently?

What would be a good way to organize the field of machine learning into sub-fields, for a new research candidate to gain a good overview of the domain. Basically what are the different streams of research active currently?

I suspect that the Wikipedia “Category:Machine_learning” [1] is as good a breakdown as any; so, because I’m interested in all applications of machine learning to dialog systems, I applied that list of sub-topics to a consistent search of Google Scholar [2].  What I can see clearly from that exercise is which sub-topics are not covered at all (unlinked) as well as the exact number of hits for each sub-topic, which gives me a good indication of the relative popularity of the various machine learning sub-topics as applied to dialog systems.

[1] http://en.wikipedia.org/wiki/Category:Machine_learning

[2] http://www.meta-guide.com/home/bibliography/google-scholar/machine-learning-dialog-systems