The 39 categories of prohibited creative work on Shabbat are called melachot in Jewish law.
The melachot are traditionally derived from the types of work associated with constructing the Tabernacle in the wilderness. They describe creative or constructive activities rather than simply physical exertion. Examples include planting, baking, writing, building, kindling a fire, and transferring objects between domains.
The categories are listed in the Mishnah, especially tractate Shabbat. Later rabbinic discussion explains their definitions, subcategories, and applications. In traditional observance, many modern activities are analyzed by asking whether they resemble or directly produce one of the categories.
Melachot should not be confused with every action that feels difficult or tiring. A strenuous activity may be permitted in some circumstances, while a small action such as writing can fall under a prohibited category. Jewish communities differ in interpretation and practical observance.