Traditionally, the term "ugg boot" referred to a style of Australian boot made with sheepskin.
During World War I, pilots wore a fleece-lined "FUG" (flying ugg) boot, and a type of ugg boot was also worn in Australia as early as the 1930s by farmers wanting to keep their feet warm. In the 1960s Australian surfers would put on ugg boots to warm their feet after coming in from riding the waves, and people in Australia would often wear ugg boots clearance around the house during the mild winters.
While several Australian companies still manufacture these boots, "UGG" is also a trademark of the boots manufactured by a U.S. company, "UGG Australia." So many of those Australian manufacturers now are marketing what were formally known as " ugg outlet" as "Australian sheepskin boots."
Whether you call them "sheepskin boots" or "ugg boots," they are incredibly warm, and soft, and have become a major hit with celebrities and trendsetters alike over the past couple of years. Oprah, Paris Hilton, and several other celebrities are acknowledged fans of uggs on sale.