Specific details of the forthcoming ad-based model were murky at the time of the announcement. Only a few months later, Netflix Co-CEO Ted Sarandos is finally giving the public a little more clarity on what to expect.

Netflix Co-CEO Ted Sarandos divulged that commercials will not be added to the current Netflix subscription plans. Instead, the new ad-based tier is intended to attract new subscribers who are not bothered by advertisements accompanying the content they’re watching.
The clarification should come as good news to current Netflix subscribers, who are currently paying either $9.99, $15.99, or $19.99 per month for the service. The prices are determined by such factors as the number of screens a subscriber chooses to watch programming on, and whether they want to watch the content on HD or Ultra HD.
Source: Netflix CEO Reveals Which Subscribers Will Have To Watch Ads (msn.com)
Edited for brevity.