auctionsAppNexus, a New York City startup that describes itself as a “real-time ad platform,” said today that it has raised $50 million in a third round of funding.

The online ad industry is now populated by a dizzying array of companies and technologies, including ad networks, ad exchanges, and “demand-side platforms”. AppNexus says it doesn’t fall into any of these categories. Instead, it offers a service where ad networks and other advertisers can bid on the ad impressions sold through exchanges (which are basically real-time auction sites for ad space) such as Google’s DoubleClick Ad Exchange and the Microsoft Advertising Exchange.

In fact, Microsoft is now an investor in AppNexus. Previous investors Venrock, Kodiak Venture Partners, and First Round Capital also participated in the round.

AppNexus said it has seen dramatic growth in the past year. It’s reportedly auctioning off 4 billion ads daily, up 676 percent from a year ago. And it serves 120 ad networks in 12 countries.