The fascia board is the long, straight board that runs along the lower edge of the roof. The fascia is fixed directly to the roof trusses and usually does all the work of supporting the bottom row of tiles and carrying all the guttering. This is no mean feat, especially when it is raining hard. In a downpour the roof of a 3-bed semi could be washing several gallons of water per second into its gutters.
The soffit board is tucked away under the fascia board. It is usually the board that you see most of from street level. The soffit can be ventilated to allow the flow of air into the roof area. Alternatively, ventilation can be provided over the top of the fascia board. Many people prefer the latter solution these days. Without adequate ventilation, condensation will form in the roof void increasing the risk of timber decay.