A multiline comment begins with a forward slash and asterisk in combination (/*), and must end with the reverse (*/). UNless commenting out a large chunk of code, most people will simply use the single line comment. When the comment runs over more than one line, each new line is started with //.
Comments allow you to annotate your script, and are useful as a reminder of what the script is doing and why. The double slash indicates a single line comment. Everything after it on the same line will be ignored by the script engine. The slash-asterisk indicates a block comment. Everything after it will be ignored by the script engine until an asterisk-slash is encountered.
Commenting out script tags
HTML comments cannot be used anywhere within your scripts. They will not be recognised, and so cause an error.