Melacha is the 39 base categories of avodah (translated, but not literally work) that went into the construction of the temple. The talmud then expands upon how we learn about and understand these - including their sources and the discussions in Mesechet Shabbat (the Sabbath Tractate).

Over the years Rabbis have clarified and expand these definitions to understand how they relate to the current generation and modern world. This is a long conversation.