The difference between metric and standard tools is that metric tools use metric measurements and standard tools use imperial measurements. Some modern tools are available in both measurements.
Standard American equivalent tools are usually referred to as standard tools or SAE tools. This system is based on imperial measurements, which means that all measurements and sizes are in inches.
Metric tools use the metric system, as the name suggests. This means that all measurements and sizes are in millimeters or centimeters.
Although there are two separate systems of measurement and sizing for tools, many tools incorporate both systems, which makes them universal. Many tool manufacturers produce tools for a variety of countries, and this keeps them from having to produce two different sets for the American and international markets. Because most other countries use the metric system and the United States has been reluctant to adopt metric measurements, having dual measurements is a good compromise.
Some sectors mostly use metric measurements, which means that some tools are more commonly produced in metric measurements only. This is true for some auto industry tools, for example. Tools produced outside of the U.S. are also more likely to come in metric measurements.
People who are unfamiliar with the metric system and cannot access standard or dual measurement tools can still use metric tools. They can simply convert the measurements to imperial using a calculator or conversion chart.