Since a lot of the users at MESE come from MSE or MO (or, more generally, have a background in Mathematics rather than Mathematics Education) I thought it might be helpful to link to the following book.
Curtis C. McKnight (Ed.). (2000). Mathematics education research: A guide for the research mathematician. American Mathematical Soc.. Link.
A review of the book can be found on the MAA website here.
From this latter link:
In a nutshell, this book is a guide for reading and evaluating mathematics education research, especially undergraduate mathematics education research. Its purpose seems to be to encourage research mathematicians, who do much of the teaching of undergraduate mathematics, to use education research to actually inform their teaching. In order to use this research, though, mathematicians need to know how to find and evaluate the research already done, and this is the focus of this book.
There is plenty more to the review, and the book itself is quite readable. (Though only part is available through the Google Books link provided above.)