Fortification help you defend against both zombies and players. Each fortification type has it's pros and cons depending on what you're trying to accomplish. For example, a full window of reinforced planks has a higher overall max HP, but will have no visibility and take a lot of materials to craft. Whereas a full window of sheet metal will have less max HP, but can provide vision and space to shoot through.
Below are all the available fortification in the game, but if you're just interested in a quick look at stat comparisons, you can click here.
Window Fortification Examples