Brewer's yeast is a type of fungus formally known as Saccharomyces cervisiae. Along with other Saccharomyces species, brewer's yeast is used to brew beer and bake some breads, and can also be used as a nutritional supplement in an inactive form. Like other yeasts, brewer's yeast will ferment carbohydrates when it comes into contact with them, forming a froth of carbon dioxide which can ferment grains into beer and cause bread to rise. When used as a dietary supplement, brewer's yeast can provide the body with a number of essential vitamins and minerals including vitamin B. While brewer's yeast can be used to bake bread, most bakers use baker's yeast specifically, a sweet and less bitter culture of Sacchraomyces.

Yeast is a living organism which forms colonies of single, simple cells classified as fungi. It grows readily as long as conditions are warm and moist, and while it eats food, it emits carbon dioxide and alcohol as byproducts, a process called fermentation. The properties of yeast were discovered thousands of years ago, in the form of floating wild yeasts, which are still used today for many artisan breads such as sourdough. Today, yeast is cultured for consistency and is readily available in a dried, ready to be activated form for making bread and beer.

A number of Saccharomyces species are used to brew beer, depending on whether it is a top or bottom fermenting beer. Some brewers use a different genus of yeast, especially for specialty beers like Belgian wheat beers. To brew beer, brewer's yeast is added to hops and malted barley and allowed to ferment the hops and barley into alcohol, made bubbly by the carbon dioxide. The beer ferments for several weeks at varying temperatures, depending on the type of beer being made, and then is ready to bottle.

In addition to being used for beer, deactivated brewer's yeast is also used as a nutritional and flavor supplement, and is often found labeled as “nutritional yeast” in grocery stores. It is important to make sure that you purchase an inactive form of yeast for a nutritional supplement, because active yeast will flourish in your intestines, potentially causing health problems. Some consumers sprinkle brewer's yeast on food simply because it tastes good, but many people, including vegetarians, also use brewer's yeast to eat a balanced diet. Brewer's yeast is extremely nutritionally rich, and contains protein, amino acids, vitamins, and minerals which can keep consumers healthy. Many folk remedies for poor skin include brewer's yeast, and some studies have suggested that consuming brewer's yeast can accelerate the healing time for cuts and similar injuries.