Zuhr is the second of the mandatory daily prayers in Islam. Zuhr begins after midday sun has passed the meridian. Zuhr ends at the start of Asr time, but it is better to have read it 15 minutes prior to the Asr prayer time.

The Zuhr prayer consists of four sunnah (recommended) prayers, four obligatory prayers, then two more sunnah prayers, and then finally two Nafl prayers. The Nafl is an extra prayer, but not mandatory. It is better to read it though, then to not read it. Many people do pray them, unless they are short on time (which really isn't a suffice reason).

To learn more about what all this means, it might be helpful to read the writeup on Salat by robwicks.

The other four prayers during the day are Fajr, Asr, Maghrib, and Isha.