Generally speaking, the dispatcher and flight crew will use standard weights for male passenegers, female passengers, children, and crew.. Standard weight figures are also used for baggage...
After filling out a loadsheet - either the dispatcher or flight crew will do this - the centre of gravity can be calculated and various types of weights ie take off weight. This is calculated by factoring in the 'weight and position of each hold, position of passangers sometimes if the a/c is fairly empty, the fuel on board etc...
Once the various weights are worked out, and by using environmental factors suchs as air temp, pressure, surface type and condition, wind component etc... various conclusions can be made - take off distrance required, accelerate stop distance if aborting at V1 etc...