Bolts are cylindrical objects with groves on the outside, which enable them to be screwed into nuts. A bolt without an end is classified as a 'lug' or 'stud'. There are many grades of nuts and bolts. Grades depend on the quality of the materials and the tolerances observed during the manufacture of the nuts and bolts. Nuts and bolts are often separated by washers.
Washers have many shapes and sizes.
They give the nut and the bolt a firm hardened surface for bedding against. The parts that you put together with nuts and bolts are usually not as strong as the nuts and bolts themselves.