It was once common to have separate main switches and fuse boxes for each circuit - lighting, the cooker, the immersion heater and, perhaps, a number for power outlets. When extra circuits were added, it was likely that an ill-assorted jumble of fittings and circuitry would result, making it unclear which circuit was which and what electrical load was being borne.
In modern practice, the compact consumer unit means that the fuse holders of correct amperage for the appliance or circuit are easily identifiable. The unit should be placed as close to the electricity meter as possible, and it is best mounted on a board to protect it from damp.
Cables are fed through the consumer unit usually through convenient 'knock-outs' - easily pushed out sections. The wires should be insulated by rubber grommets located in these holes to prevent the sheath covering the inner wire core from chaffing. One entry hole is used for the circuit wiring, and another for the meter 'tails', which are connected to your electricity board's meter.
Unless you are extremely confident about doing electrical work, this job should be left for a qualified electrician. In any case, you will have to have your unit inspected by the Electricity Board before it is connected to the mains.
Safety Tip
Wiring a consumer unit is a professional job which should only be undertaken by experts and is not recommended for you to try for yourself. However, if you do decide that you do have the expertise to carry this work out yourself, after installing the new consumer unit, you MUST arrange for the Electricity Board to test the circuits and to connect the unit to the meter and the earth. Connection of the consumer unit to the meter and earth may only be done by your Electricity Board. Do not make these connections yourself under any circumstances.
