Sill is the section that the windows sits on and the main idea of it is to take the water away from the bottom of the window and the wall. Traditionally, sills are made of concrete but in the recent years PVC window manufacturers have also been able to add a PVC sill to the windows if it is required. The main thing with installing sills is to make sure that they are waterproofed from all directions by placing waterproofing material such as DPC all around them and ensuring that they are angled away from the house so that the water can float away.
The function of a lintel is to span the opening in a wall and carry the weight of the masonry above doors and windows. This is done to make sure that there is no weight load on doors and windows. In the old days a very big and strong piece of timber was used as lintels, but the problem with these were that after a while they might have suffered from dry rot or wood worms and they would give way to the load above ending up with cracks on walls or damaged door and window frames.
Today, there are mainly two types of lintels which are used in homes. They are the precast concrete lintels and pressed metal lintels. If metal lintels are used they will have to be installed according to the manufacturers instructions as far as fire and thermal properties, water resistance, and corrosion protection.
Do remember, if the length of a lintel exceeds 3 meters long your architect or engineer has to specify the thickness and the measurement required.
It is important that you order the lintels ahead of the time as you do not wish to be delayed by them. If the block layers do not have the lintels on site they can progress any further therefore you could end up with delays.