The most popular option, you can choose to have the IRS payment taken directly from a bank account. You just enter in the routing number and bank account number. Just double check and make sure the numbers are correct before you transmit your return to the IRS.
An EFTPS account is a tax payment service provided by the US Dept of Treasury. You can enroll in this service for free and once you receive your credentials, you can pay any tax due to the IRS using this system. You will not choose this option if you do not have an EFTPS account set up beforehand. Visit EFTPS.gov for more info on setting up this type of account.
This is great option if you are hesitant about using your bank routing and account number. The first option is more convenient, but some people still like doing things old school. If you choose this option, you will be emailed a payment voucher after you transmit your return to the IRS. Then all you will do is print out the payment voucher and mail it in with the check/money order. The voucher will have all the directions for what to put where. You want to have the check/money order in the mail as soon as possible, but no later than a week after transmitting the return. Make sure to hold on to the money order stub just in case you need to verify you sent the payment, always good to have some insurance.