I was not much adept on how to answer it

Hi. The impact is so great and far-reaching that it would be best to answer this by focusing on just a few or even two of the major impacts of online advertising.

The ability to measure views and clicks is one very big impact. It is difficult - if not impossible - to measure traditional advertising. Do a little research on this and you'll be able to put together a nice 30 - 60 second answer just on measurability alone.

Another area could be in the social media arena of reputation management. The impact of consumer ratings and reviews is huge today. More than ever everyone from grade school students to grandmothers are online. They are interacting with online networks that they have build and it isn't just their "real-life" friends and families in those networks. If you're a business and you have just a few bad reviews online - that bad review can spread like wild-fire causing a negative impact on any advertising campaign.

I hope that helps. Any time a question is asked like this one during an interview; a question that is so broad - always try to break it down into just a couple aspects and speak in depth to those. That will likely be all the interviewer is looking for. And remember there is nothing wrong in taking 30 - 60 seconds to reflect on the question before answering. Say something like, "hmmm, that is an interesting question, let me think...".

Good luck!