In practice, the difference is in the location where the preprocessor searches for the included file.
For #include <filename> the C preprocessor looks for the filename in the predefined list of system directories first and then to the directories told by the user(we can use -I option to add directories to the mentioned predefined list).
For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.
