What is NAV? How often is NAV announced?

A mutual fund usually from all the money that it collects from its investors would invest it in various assets. All the assets put together, the market value of that divided by the number of units outstanding on a particular day gives a net asset value of that unit on a daily basis. The NAV of a mutual fund fluctuates on a daily basis therefore in case of an open ended mutual fund you would have the NAVs being declared on a daily basis but for a close ended mutual fund this is declared only on a weekly basis.