In easy words, it allows you to purchase stuff in easy monthly instalments. That is, wherever you saw an EMI option during buying something, they all work with only credit cards. The way it works is that your bank pays the money on your behalf, and you pay the bank in small amounts every month. The other use of credit cards is to pay for any item as you would do with a debit card. Now, where the above description applies and where the below one is a matter of choice and requirement.
What kind of choice and requirements?
To begin with, credit cards are used to make purchases beyond your account credit. For example, you have 100 bucks, and you are trying to purchase something worth 1000, you could ask your bank to break it down into 3 or 6 or 12 months of EMI, and you would give around 100 for 10 months and maybe a few bucks more depending on the rate of interest. In many cases, though, especially electronics, you have up to 12 months of zero percent EMI, and so on. But, these are various offers and special cases. Thus, it is a matter of choice and requirement in this case.
Specifically, you can choose the number of months you want to break that amount into. Usually, the number of month’s means, the number of times you want to break the money into. So, 1000 bucks divided into 3 months would roughly mean around 300 bucks per month and so on.
The requirement has to spend money beyond your account credit and your other monthly expenses. If you are not able to bear spending 1000 bucks all at once and left with nothing to eat the rest of the month, better prefer EMIs.
There is a notion of a grace period too.