The name of the project does not matter. The nitty gritty details of how the project fit in to the company's business model do not matter. The technical aspects of what you did are what the company is looking for.
For instance, let's say I was working on Google's search algorithm and not allowed to disclose it.
I am currently working on an algorithm named PageRank which creates search results based on a link weighting system. Each page on the internet is given a ranking based on how many other pages link to it. So if a page has many other pages linking to it, it is seen as more reliable. And if a more reliable page links to another page, that page is seen more reliable by association. This way we are able to create a better web search that allows ranking to be based on something akin to reputation rather than keywords.
The above description is a great explanation of what the company is developing, but doesn't say anything about what I'm actually doing.
I am currently working on a complex algorithm in PASCAL that analyzes over a trillion data points that maps the relationships between those points to assign a value to each.
This description focuses on what I'm doing without giving away any trade secrets whatsoever.
Having a non-disclosure clause in your contract is something you should respect, but it doesn't mean that you can't discuss in general terms what you are working on or what problems you are tackling.
Use common sense and just ask yourself, "If I shared this information, would it harm the company I'm working for?" The second description shouldn't, and should be enough for your prospective employer. If during the interview they ask for more specific details which you are uncomfortable sharing, just explain something like:
I can tell you that the data points I am working with are a network of linked items (think friends on Facebook), but due to my non-disclosure agreement I can't tell you the specifics of what we are actually analyzing.
At the end of the day, if you can't explain what you are doing without explaining what the overall project is, you probably aren't going to have a very good interview.