What is the difference between the normal comments and the document comments?

Answer 1

Info that the user of a class needs to know should go into a Javadoc comment.
Info that a developer modifying a class needs to know go into a normal comment (block or line).

And it's very possible that any block of code can have both a Javadoc comment and a normal comment block, when both publicly visible as well as internal documentaton is required.