//, /*...*/ | NN 2 IE 3 ECMA 1
These are comment statements that let you enter nonexecuting text in a script. Any text following the // symbol anywhere in a statement line is ignored by the language interpreter. The next line of script, unless it begins with another // symbol, is interpreted by the browser.
For multiline comment blocks, you can begin a block with the /* symbol. Comment blocks may run any number of lines. The block is closed with the */ symbol, after which the interpreter engages subsequent statements.
Example
    // convert temp from C to F

    /*
    many lines
    of
    comments
    */