TL;DR: The easiest way to find out if you have a "proper" credit card is to ask your bank, rather than the internet at large. (Also, Hertz will only know when they try).
What is the difference?
A credit card is card which has its own credit account. Basically the credit card gives you a line of credit (e.g. 1000 EUR). When you spend 100 EUR, the credit card company pays the merchant, and you then owe the credit card company 100 EUR. You have to pay back that amount eventually, for example at the end of the month.
A debit card is a plastic card that is directly connected to your bank account. When you pay 100 EUR, they are taken out of your bank account and that's it (there are also debit cards where you put in money in advance and spend it later).
What with the car rental?
Car rental companies use a feature called an authorization hold.
This works mainly with credit cards, and it "blocks" the amount on the card, without actually taking it. For example, when your collateral is 500 EUR, the bank holds that amount.
This means that while no money is paid, the credit card promises to pay 500 EUR to the car rental if asked to. The credit card company reduces your credit limit by 500 EUR.
If you return the car undamaged, the rental company releases the block. If you damage it, they take the 500 EUR and you have to pay.
In theory you can also place such a hold on a debit card, but only for a shorter time. The amount would be immediately booked against your checking account, as the card does not have its separate account.
Car rental companies often do not accept debit cards, because the hold can expire before the rental is completed. (As explained in the linked article, the company may also require a credit card to ensure that the customer has a good credit rating).
Some rental companies, at least in Europe, will accept a cash collateral in lieu of a credit card, though.
How to tell the difference
This is where the confusing part starts: Credit and Debit cards are often hard to tell apart. The big payment networks (like Visa or Mastercard) have both credit and debit cards. They work almost exactly the same - except when they don't, like for the car rental.
- Most importantly: Your bank will know. They know the difference between debit and credit cards, and which one they gave you.
- A card where you pay once a month (or later), or when you decide when to pay back, it is certainly a credit card. However, the reverse is not always true: I have a "proper" credit card, where the amount is paid back a day after each purchase.
- You cannot tell the difference by card number or color. There are some visual indications when the card is a debit card, but since the bank have much freedom in designing the cards you cannot always tell.
- When the card has its own credit limit and account number, it is likely a credit card. However, debit cards can also have daily spending limits, so that is easy to confuse.
- "Going below 0" does not mean a credit card. If you have a line of credit on your bank account, it can go below 0 from a debit card payment as well as from a credit card payment. The important part if the card has a line of credit which is separate from your regular bank account.