Tool
Throughout the development of mankind, there has always been a need to complete a task, whether it be to start a fire, hunt for food, or build a house. These necessary tasks for survival have driven a fundamental want for a way to accomplish these tasks in an easier, faster, more efficient way. Thus the requirement for tools was born, and since the very early days of thought there were always discoveries that lead to the development of more complicated tools.
Early tools
Some of the very early primitive tools were probably rocks or sticks that had sharp edges or points. The lethality or effectiveness of these edges and points gave early man an advantage; suddenly he could kill his prey more quickly, and cut into the skin to get at the meat, or land a killing bludgeon on his opponent. Furthermore, seeing two rocks collide he could have observed a spark, creating excitement that it could spurn fire, enabling him to provide much needed warmth for survival. There is evidence that early man used these "tools" for survival in caves that have been discovered which inside contain drawings and carvings of humans using sticks and spears.
Modern tools
Almost all modern tools are constructed to follow standards in size, functionality, utility, and shape. The key differences usually amount to the variable forces that can be produced by the tool, such as torque, pressure, rotation speed, impact, etc. Ergonomics are also a major consideration in modern tool design; there are ways to design tools which allow the user to impart a maximum amount of force with minimal impact stress on muscles and bone structures. There is a premium cost usually associated with these designs.
Metric vs. Standard
Many tools today are able to be adjusted to perform work on parts that are either metric-sized or standard sized. All metric sizes for tools are in meters, whereas standard sizes are represented by a number(3/8 mm versus a "4").