Just before each class/interface/constructor/field/method:
/**
* Class/field/method does this...
*/
Note the /** start. The * on each subsequent line marks the start of text to output, for alignment, but is not itself output. The text of the comment is HTML content: can use <p>, <b>, <li>, etc.
Special javadoc tags (there are others, see API docs):
@param paramname description
@return description
@author name } only for /** comments before class
@version version-text }
{@link package.class#member label-text}