Comments are mainly used in javascripts to help the programmer(s) to find steps in the coding of the script. It is usually to identify certain functions and/or processes within the coding of a javascript. The comments tags are invisible to users of a web page unless they view the source code (Click here) of the HTML page they are at.
As this information is invisible to the user unless you view the source of the page, many people have tried to use this method to stuff keywords into their pages. The spamming technique uses the comments tag to stuff the webpage with keywords.
Comments tags if used properly won't be penalised against. Although there is a fine line between right and wrong. If you have a large amount of comments on your page you may be finding that search engines will not index your page, even if you are using them properly. Spiders will generally make up their own mind. So try and limit comment usage on your page, and keep them strictly for javascripts to try and avoid this problem.