Can I use my debit card to purchase stuff online?

I was given a debit card from my bank and I was told that I can use it as a credit card too. It looks exactly like a credit card, it's a Mastercard, it has 16 digit number and an expiration date as well. I used it in several retail stores as credit and the money was taken away from my bank account. I am... show more I was given a debit card from my bank and I was told that I can use it as a credit card too. It looks exactly like a credit card, it's a Mastercard, it has 16 digit number and an expiration date as well. I used it in several retail stores as credit and the money was taken away from my bank account.

I am confused whether I can use it online to buy softwares etc. Please help, I saw a great deal online for limited time and I want to buy it.
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