First one is for Javadoc you define on the top of classes, interfaces, methods etc. You can use Javadoc as the name suggest to document your code on what the class does or what method does etc and generate report on it.
Second one is code block comment. Say for example you have some code block which you do not want compiler to interpret then you use code block comment.
another one is // this you use on statement level to specify what the proceeding lines of codes are supposed to do.
There are some other also like //TODO, this will mark that you want to do something later on that place
//FIXME you can use when you have some temporary solution but you want to visit later and make it better.
Hope this helps