Make sure you save the form at the end and take note of the reference number assigned to you - it is handy to have it everytime you enter the US as you need to update the flight arrival and the place you are staying each time. I got my approval before they introduced the credit card charge so it may have changed but previously they did not send you any details, it was up to you to retain them.
The ESTA system is, to put it mildly, clunky, but at least now you don't also have to go the bother of filling out that little green form they used to give you as well. If you think it annoying that the airline and the US government both ask you to submit your details, you should have tried it when additionally you also had to fill out the customs form (which you still have to do) AND that green landing card thingy too with pretty much so the same info.
Your ESTA is valid for two years and as many entries to the US as you like, each flight landing into the US must provide the CBP with specific passenger details (including things like meal choice if you request a special meal). The function of each is slightly different. ESTA is because you do not have a US passport but do not need a US visa for a visit - so they need to know who you are. The airline API applies to all passengers irrespective of their citizenship or visa status etc. and is to screen for potential threats landing into the country. Hence the reason why they need those details in advance also. The API can be done at the airport but the ESTA must be done in advance.
Another thing is - if you are going to the US a credit card would be advisable - you can survive without one but they do tend to be very useful!