The following information is freely available and taken together is an excellent introduction to the subject.
Jason Maassen, C for Java Programmers. A complete introduction to C for Java people. I have used and extended his slides for this lecture but you should read the background information here as well.60+ pages. Good if you need to prepare for a test or need some more information about a feature from my slides. Find his C course with other materials here: http://www.cs.vu.nl/~jason/course.html .
Steven Simpson, Learning C from Java. An experienced Java programmer will get the most from this short paper focussing on the differences. Excellent. http://www.comp.lancs.ac.uk/computing/users/ss/java2c/diffs.htm .
Marshal Brain, How C programming works. Another excellent paper from www.howstuffworks.com. This really explains complicated memory problems using pointers, how array overwriting can happen etc. And many useful pointers to other computing related topics like memory organization, operating systems etc. www.howstuffworks.com