HMRC wanted account number? Watch
Card number for standing orders.
The difference is that with a direct debt you are effectivly 'pushing' the money out of your account and into theirs. With a standing order they are 'pulling' the money out of your account. It means you have control over the amounts sent in a direct debit however the company 'pulling' the money in a standing order can change the amount. It's also much easier to cancel a direct debit as you have control over it.