For example, Air France Flight 447 crashed into the ocean, but what gives it the tag of 447? Air France has had a lot more flights than that. Is it the flight number on a specific day? What then happens if another 447th plane-of-the-day crashes later? Is it given the tag 447B?
I thought I'd expand on the brief information we have.
The number is the 'flight number' of the aircraft, colloquially, although the official IATA name is 'flight code'. It is assigned to a particular route from city to city. For example, NZ1 could be London (LHR) to Los Angeles (LAX) to Auckland (AKL) on Air New Zealand.
Now, there are some guidelines on the numbers that get used. However, these will vary from airline to airline.
And as someone mentioned, in the event of a major incident/crash, the flight number is often retired/changed. American Airlines Flight 77, which regularly flew from Dulles International Airport in Washington, DC, to Los Angeles International Airport, was changed to Flight 149 after the September 11, 2001 attacks.
Finally, a bit off topic, but flight codes are also sometimes used for spacecraft, eg Ariane 5 Flight 501, although since they're expendable, they generally only get used once. Space Shuttle launches used to get numbers with the prefix STS, for example, STS-121.
It is the regularly scheduled flight number. For example, Qantas flight 8 may refer to their Tuesday flight from LAX to Sydney.
Once a flight results in an accident with fatalities, the flight number is often retired and not used again by the same airline.