Metric fasteners are fasteners that are manufactured and labeled according to a worldwide standard of measurement. That worldwide standard is set by the International Organization for Standardization (ISO which is based in Geneva, Switzerland. That worldwide standard is based on the metric system.
There are many ways in which fasteners can be measured and manufactured for size. It is important that there be some kind of standard for screws, with the countless number of products and equipment that rely on screws to stay fast together. The United Kingdom, United States, and Canada invented as faster measurement system in 1949 called the Unified Thread Standard. They hoped it would become the worldwide standard. But the ISO standards eventually won out. The UK no longer uses that standard at all, and Canada is currently midway through the transition to the ISO metric fastener standard. Today, 40% of screws and products made in the United States are made with metric faster systems. Gradually, metric fasteners are replacing UTS fasteners. Soon, UTS will be extinct.
Metric fasteners are measured in millimeters by their thread diameter from edge to edge, and not by the shank portion of the screw. The capital letter M will precede the number of millimeters wide the thread is. For example, a metric fastener can be 5 millimeters wide, and it will be denoted as M5. A thread 6 millimeters wide is denoted as M6.
Metric fasteners used to come with two levels of thread thickness: fine and coarse. Neither one was better or worse than the other, but they are not compatible. So the ISO decided on fine-threaded screws as the standard. Today, most new products are made with fine threads in mind, but you can still get coarse threads, since some older products that use coarse threads still need to be repaired from time to time.