There are five daily obligatory prayers. These are the dusk prayer (Maghrib), night prayer (Ishaa), dawn prayer (Fajer), noon prayer (Zuher), and the afternoon prayer (Asr). Besides these there are a few other prayers that should be performed only on certain occasions, but not regularly, like the prayer at the burial ceremony of a Muslim who has passed away, the prayer on two specific Islmaic festivals (called "Eid"), etc.
Other than these, one can pray whenever he or she desires, but these are counted as supererogatory prayers. Basically, Muslims must offer the five daily prayers (these are obligatory), but there is no limit to how much extra a person can pray.
source : sibtayn.com
