Ugg boots are made from sheepskins with fleece attached. The fleece is tanned into the leather and the boot is assembled with the fleece on the inside. Ugg boots often have a synthetic sole, commonly made from Ethylene-vinyl acetate (or EVA) and the stitching is often prominent on the outside of the boot. The natural insulative properties of sheepskin gives thermostatic properties to the boots: the thick fleecy fibers on the inner part of the boots wick moisture and allow air to circulate, keeping the feet at body temperature. This property allows Ugg boots to keep feet warm in cold weather and cool in hot weather. The original design was a pull-on boot in natural (undyed) tan sheepskin, about 10 inches (25 cm) in height, with rounded, almost shapeless uppers; this is now described as the “classic” design. Produced by a number of manufacturers, they come in a variety of colours, including black, pink, blue, chestnut, and fuchsia. They are available in both pull-on and lace-up varieties and their height can range from just above the ankle to above the knee.

Some variations of ugg style boots have also been made from kangaroo fur and leather. There are also synthetic boots. Although derided as “fake” by some in the industry, their lower price made them appealing to large retail chains such as Myer.

In 2009 American podiatrist Ed Chairman raised concern that the regular wearing of ugg boots could be deleterious to foot health due to the lack of arch support.

The origins of the ugg boot style are disputed, with both Australia and New Zealand claiming to have been the origin of the footwear. Sheepskin boots were known in rural Australia during the 1920s. While it is not clear when manufacturing started, by 1933 sheepskin boots were being manufactured by Blue Mountains Ugg Boots, and Mortels Sheepskin Factory was making the boots from the late 1950s.

In regard to naming, it appears that ugg boots, ugh boots and ug boots have been used as descriptive terms for sheepskin boots in Australia and New Zealand since at least the 1960s,although individual accounts have suggested that the terms (or variations thereof) were employed earlier – for example, “fug boots” were worn by pilots during World War I. The 1970s saw the emergence of advertising using the names, but Brian Smith has stated that the boots were referred to as “uggs” long before the word was trademarked. Frank Mortel of Mortels Sheepskin Factory claims he named them “ugg boots” in 1958, when Mortels started production, after his wife commented that the first pair he made were “ugly.”

In the 1960s, ugg boots became a popular option for competitive surfers, who used the boots to keep their feet warm after exiting from the surf. Surfing helped popularise the boots outside Australia and New Zealand, when Brian Smith started selling the boots in the United States through the company Ugg Holdings, Inc. in 1979. Ugg Holdings was sold to Deckers Outdoor Corporation in 1995. Generally worn for warmth and comfort, ugg boots had never been considered fashionable, but in the late 1990s and early 2000s sheepskin boots emerged as a fashion trend in the US through Deckers’ promotions of the UGG brand, with celebrities such as Kate Hudson, Sarah Jessica Parker, Cameron Diaz, Leonardo DiCaprio and Jennifer Lopez wearing the boots, and with Oprah Winfrey featured UGG brand boots as one of her “Favorite Things” on her TV talk show. Deckers’ actions to promote their product “led to an exponential growth in the brand’s popularity and recognizability” with the company reporting close to a 50-fold increase in sales from 1995 to 2008.