Hilchot Shabbat ch. 7 – Avot Melacha and names of Avot
Labor on Shabbat is divided into 39 Avot, or categories. For example, plowing and planting. However, the Avot are not these actions themselves, but rather the category which these are representative of. Thus “Choresh”, plowing, is any work involving excavation. The names of the Avot are typical examples, but they are distinct from the Categories themselves, each of which needs definition. For example, planting is name for the category of causing plants to grow, even though they are distinct physical behaviors (planting a seed, grafting etc.). This is an example of a general point about language, when we need to distinguish the names of things from the things themselves.
These categories are not only based on the structure of the action, but rather the intention as well. This is because the concept of labor on shabbat is not based on difficulty of work, but rather “Melechet Machshevet”, creative, intentional productivity.
The categories also have derivatives, Toldot, which are also forbidden from the Torah. These are cases which are similar to the primary category. But I am still unclear on the line between Avot and Toldot.