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.
- East and north-bound flights usually get even numbers
- West and south-bound flights usually get odd numbers
- Some other airlines will use an odd number for the outbound flight, and even for the inbound
- If a flight repeats between cities on the same day, the numbers will tend to increase - eg 235/236, followed by 237/238.
- Long haul tends to be less than 3 digits, and '1' is usually their 'flagship' flight - like the NZ1 I referred to earlier. BA1 used to be their Concorde flight from London to New York!
- four digit up to 4999 are usually regional affiliate flights
- 5000 and up will be usually codeshare flights (or even railways!!!)
- Above 9000 is special - a 'ferry' flight (no passengers) to relocate a plane.
- Flights starting with an 8 are sometimes charter flights
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.