With advanced segmentation, you can look beyond the totals and into the nuances of the data for your site. For example, the average time on site for all visits could be 60 seconds, but when you segment by country, you might learn that average time on site of visits from Poland is over 2 minutes.

So we’ve added two new ways to use advanced segments through the API:

1. Create them on the fly by specifying their expression directly through an API query.

2. Use advanced segments created in the Google Analytics web interface through the API.

This video describes exactly what advanced segments do and how you can use them with the API.

via Google Code Blog: New Google Analytics API Features.