Just to add some information for Bio15...this is my knowledge based on certain carriers, and I'm not claiming this is authoritative (ie. please correct any inaccuracies if you find them).
Weight build-up generally follows these steps:
* Start with the Basic Weight of the aircraft. The Basic Weight is the weight of the empty aircraft as weighed by the manufacturer. The airline might make structural changes or other changes like adding galleys and so on. All these changes must be included in the Basic Weight (and index, obviously, but I won't go into index effects here as the initial question was about weights only). This Basic Weight includes seats, cabin fittings etc.
* Add the weight of any service weight adjustments, such as stretchers, flight spares, etc.
* Add the weight of crew, crew bags and pantry.
* Add the weight of any trapped or ballast fuel. This is fuel that will not be used.
* You now have what is known as the Dry Operating Weight. (DOW).
* To the DOW, add the weight of deadload (cargo, pax bags, etc) and add the weight of the pax themselves. Early in the piece you'll have estimated figures, but as the pax start checking themselves and their bags in, you'll start to get actual data.
* You now have the Zero Fuel Weight (ZFW). There will be a maximum ZFW (MZFW) as defined by the manufacturer. Obviously you should check your ZFW is less than your MZFW.
* To the ZFW add the usable fuel weight, including taxy fuel. You now have a Ramp Weight (also sometimes called Taxy Weight).
* The aircraft will burn some fuel when taxying. So the Takeoff Weight = Ramp Weight - Weight of Taxy Fuel burned. Usually this is an approximation, but the amount of Taxy fuel allowance might change depending on the airport. Some large airports require a long taxy.
Again there is a manufacturer or carrier imposed Maximum Takeoff Weight (MTOW) which the actual TOW should be less than.
* During flight, the trip fuel will burn off. So your Landing Weight will usually equal your Takeoff weight minus trip fuel burned. There are too many variables in fuel burn to really accurately calculate this before the flight (ie. on the ground before departure). It's usually worked out by the FMC as that will have fairly accurate fuel usage information as the aircraft burns fuel off during flight. The way the FMC might work it out is ZFW plus the weight of remaining fuel in tanks at landing. I'm not sure of the workings of FMCs so I won't comment, but ideally these two calculations will produce the same result
* Again there will be an imposed Maximum Landing Weight which ideally should not be exceeded. However in some emergency situations where there hasn't been enough time to dump fuel, the aircraft may have to land over it's max landing weight. This may put excessive stress on the undercarriage and other structures, as well as brakes / tyres.