Rabbi Jack Abramowitz is Torah Content Editor at the Orthodox Union. He is the author of five books, including The Tzniyus Book. His latest work, The Taryag Companion, is available from OU Press as well as on Amazon.
There are 39 categories of constructive labor called melachos. These are the acts that were performed in the construction of the Mishkan (Tabernacle). These and similar actions are forbidden on Shabbos.