Yeast
Yeast is a single-celled fungus that converts sugars into carbon dioxide and alcohol. The gas, trapped by the gluten network, is what lifts the dough. Baking relies mainly on the species Saccharomyces cerevisiae.
Commercial yeast comes in three forms. Fresh, or compressed, is around 70% water, keeps only weeks refrigerated and needs dissolving first. Active dry needs rehydrating in warm water. Instant goes straight into the flour, and is the most concentrated and convenient.
Conversion between them runs roughly: instant is a third the weight of fresh, active dry about half.
The wild yeasts in a sourdough starter are different species, often from the genus Kazachstania. They work more slowly than commercial yeast and coexist with lactic acid bacteria, which is what gives sourdough its distinct flavour and keeping quality.
Yeast is inhibited by salt and by high sugar concentrations, so enriched doughs ferment more slowly, and salt is never added directly onto yeast.