can i use a cash card to buy online?

If you are referring to a visa/mastercard gift card which carries a positive balance, then yes, you can use it anywhere they accept the credit card.

If you are referring to something else, then the merchant will have to accept whatever it is you are offering to pay with. But I've never really heard of a "cash card" that isn't a visa/mc gift card.

Sometimes a cash card and debit card are one and the same thing but some banks issue both separately. Typically a cash card is only used for withdrawing money from a cash machine and will not have a VISA logo or the square foil 'watermark'. You usually do not sign a cash card either. A debit card does have these logos and will have a signature strip on the back which will also have some numbers printed on. You can't use a cash card to buy online. If it doesn't say VISA or MAESTRO, and there isn't numbers on the signature strip (the last three will stand alone and form your security code) you can't use it to buy online