The 320th prohibition is that we are forbidden to do melachah1 on Shabbos.

The source of this commandment is G‑d's statement2, ["It is Shabbos to the L‑rd, your G‑d;] do not do any melachah."

If the act was intentional, but the court has insufficient proof, Scripture3 specifies the punishment as kares.4 If the act was intentional, and there is sufficient proof,5 the punishment is execution by s'kilah.6 If the act was unintentional, he must bring a sin-offering.7

The details of this commandment are discussed in the tractate Shabbos.