Fuel requirements vary depending upon whether you are at the planning stage or in actual flight. Different airlines will have their own planning mothods but typically it's:
A fixed reserve - 30mins
Approach fuel - to get from 1,500' to end of the landing roll
Flight fuel - from the start of take off roll to 1,500' at destination.
Taxi fuel - from the gate to the end of the runway.
Variable reserve - 10% of the flight fuel
Holding fuel - for nominated ATC holding.
WX holding fuel - to cover any TEMPO or INTER where the weather is below the nominated holding criteria.
Diversion fuel - if the WX holding periods exceed the ability to hold.
When in flight things change and all you need is:
1) Fuel to go somewhere (anywhere)
2) 10% of (1)
3) Approach fuel
4) Minimum reserve
5) WX holding AND but not always, ATC holding
6) Alternate fuel, but only if you can't manage (5)