Paytm is a today biggest online portal for recharging mobile, DTH and bill payments. They have offered various segments for users and each recharge and bill payment have a paytm cashback offers if you recharge till mentioned amount.
You can navigate the link to see paytm cashback offers https://paytm.com/offer/recharge where all cash back offers have been given. To avail these offers, you must have an account in the paytm and then try to recharge with the same amount and your cashback will be credited in Paytm wallet. You can use later this wallet for further recharge, online payment, shopping through Paytm website.
If you want to learn how to create a Paytm account, then go to this link http://makemyvision.com/signin-paytm-and-prepaid-mobile-recharge/771
Learn how to recharge mobile and DTH online uses Paytm use this link