This option should contain a semicolon separated list of class titles and class names separated by =. The titles are the ones that get presented to the user in the styles drop down list and and the class names is the classes that gets inserted. If this option isn't defined TinyMCE auto imports the classes from the content CSS.
Example of usage of the theme_advanced_styles option:
tinyMCE.init({ ... theme_advanced_styles : "Header 1=header1;Header 2=header2;Header 3=header3;Table Row=tableRow1" });