This formula nests the AND function inside the OR function to search for units sold between April 1, 2011 and January 1, 2012, or any units sold by Nancy. You can see it returns True for units sold by Nancy, and also for units sold by Tim and Ed during the dates specified in the formula. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.