Salt
Most bread contains a small amount of salt, usually around 1-2% of the flour weight. The obvious reason for adding salt is to make the bread taste nice, but salt also plays a vital role in strengthening the gluten network allowing the bread to rise well. Salt is especially important in sourdough or wild yeast bread where it limits the natural bacterial activity in the dough and prevents it from causing too much damage to the gluten.
One notable bread which does not contain salt is the Italian Pane Toscano or Tuscan bread. The people of Tuscany rebelled against a tax placed on salt in the thirteenth or fourteenth century by refusing to buy salt, and the tradition of making saltless bread continues in the region to this day. The bread is often used to accompany salty foods such as cheese, anchovies, or cured meats, so the blandness of the bread is not considered to be a problem.
Recipes on this site use measurements of salt in teaspoons. A level teaspoon of salt weighs 6g - this is too small an amount for many kitchen scales to measure accurately, so it makes sense to measure by volume. If you prefer to use coarse crystal rock salt or sea salt, you should always dissolve the salt in water (taken from the water you have measured out for the recipe) before adding it to the flour.
Many bakers mix the flour and water together and allow them to stand for 30 minutes before adding salt and yeast and kneading. This "autolysis" method is believed to make the dough easier to handle and to require less kneading.