Some websites ask you for your account information like your user name and password from another provider.
An example is TNT Turner Network Television website. They ask you for your service provider account user name and password before accessing their website to watch videos.
You never ever give out this information. With this information they will have access to your cable account and can view all your account settings and change anything on your account. You could get service changes that you never know about.
They may have a disclaimer about they will not use the information but never trust it.
No legitimate secure website would ever ask for this personal information, so if they ever do move on to another site and warn your friends as well about the site you found.